Nice way to investigate mass transport limitations in non-homogeneous particles. However, there are problems with the presentation and analysis:

1. Can you show H2O/D2O exchange for just aqueous AS (or LiCl if efflorescence is an issue)? This would give some idea of the response time in your cell when switching between H2O and D2O. For instance, the characteristic time reported in Fig. 3 is probably just the time required for the cell to switch between H2O and D2O.

2. Throughout the paper you have this issue with \phi_{OD} not going to 0 or 1 as the experiment time becomes very long. This leads to some fairly implausible proposals (in my opinion) towards the end of the discussion. Isn't it simply the well-known issue that strong WGMs heavily distort vibrational bands, making their use in quantitative work difficult? The classic example is Anal. Chem. 2005, 77, 7148-7155 where the authors, who had previously been using an EDB, switched to glass slides because of this very issue (they state this in the introduction to that paper).

Although I suppose the argument given on lines 205-207 that there is condensation of the species being replaced somewhere on the walls is also possible.

3. Can you list the fitted droplet size where applicable, e.g. in Fig. 3.

4. Why are the WGMs shifting in Fig. 3. Isn't the droplet in equilibrium with the surroundings? Is some other parameter changing?

5. What are the uncertainties associated with the measured \tau reported in Table 1? Were any of these measurements repeated more than once?

Awkward writing:

"surplus protons are considered to have appreciable impacts..." This is a weird way of saying low pH. 

"panoramically presents a panorama"

"equilibrated in D2O in ambiance"

This manuscript reports measurements of water diffusion in homogeneous and phase-separated particles using an isotope exchange method in conjunction with Raman spectroscopy in an aerosol optical tweezers (AOT). This method was first demonstrated in an AOT by Davies and Wilson (https://doi.org/10.1021/acs.analchem.5b04315) and subsequently in an electrodynamic balance (EDB) by Nadler et al. (https://doi.org/10.1039/C8CP07052K). In the current work, the authors describe the application of the technique towards studying water diffusion in phase separated particles - ammonium sulfate solution droplets partially engulfed by oleic acid, and core shell ammonium sulfate droplets coated with diethyl-L-tartrate and 1,2,6-hexanetriol. The results indicate both forms of phase separation lead to slow water diffusion, with core-shell particles exhibiting the slowest rate of water transport. This work is interesting and there are clear motivations towards exploring the physical chemistry of phase separated particles. The figures are well made and clarify key points of discussion from the text. However, in its current form, I do not think the manuscript conclusively demonstrates water diffusion limitations are responsible for the slow exchange. My comments below include several suggestions for additional commentary, clarifications, and possible measurements that would strengthen the arguments made in the manuscript. 

1. The figures showing the isotope exchange (Figure 3 in particular) indicate the WGM’s move significantly during the process. Is this due to a change in the RH when switching between H2O and D2O? The SI indicates some correction to account for this, but it is unclear how effective this method is given the observations. 
2. The WGM’s in the spectra are very broad, up to 1 nm in width. These features on particles of the size reported should be narrow, <<1 nm. Typically, in AOT systems, spectrometer resolution of <0.1nm are used to resolve 1st order modes. Can the authors confirm the grating and the resolution of their spectrometer? 
3. The spectrum for a core-shell particle will depend on the thickness of the shell, and whether the WGM’s penetrate deeply enough to interfere with the internal interface. A core-shell particle retains spherical symmetry, so while the position of the WGM’s in the spectrum may differ from a homogeneous particle, the peaks generally should appear well resolved. A deviation for spherical symmetry would lead to the spectrum degrading, which may occur if the shell thickness is not uniform. 
4. Were diffusion coefficients measured at lower RH for a well-characterized system, such as CA, to validate the accuracy against previous data? 
5. What is the chamber response time? How much does this vary due to the nebulization process depositing more material into the chamber? Was the chamber cleaned out between measurements?
6. How was the oleic acid mixture nebulized, given the insoluble nature of oleic acid? How much OA was in the levitated particles? Previously, OA has been introduced as a vapor into AOT’s and allowed to condensed on existing particles (such as NaCl, in the work of Dennis-Smither et al. (https://doi.org/10.1029/2012JD018163)). 
7. Were measurements performed for water diffusion in 1,2,6-hexanetriol alone, under the same RH conditions? Based on its hygroscopicity and fluidity, it seems unlikely major diffusion limitations would be observed for water in this system. Does the presence of ammonium sulfate in particles in the present work lead to some kind of change in the rheology that leads to more diffusion limitations?
8. How much oleic acid is present in the particles? From the SI figure, the coating is small compared to the size of the droplet and the reduced surface area of the aqueous phase relative to a pure aqueous droplet would be small. 
9. Table 1 indicates the addition of acid leads to homogeneous morphology, but the rate of diffusion is still much slower than for a homogeneous system. Further the influence of lowered pH in this system would lead to protonated organic molecules that would be less polar, rather than more polar, suggesting miscibility should decrease rather than increase. Perhaps the pH influences the proton exchanges rates on the alcohol groups, which affects the timescales?
10. For the core-shell fitting, can the authors provide simulated spectra to compare with the measured core-shell spectra? More details on the use of this and the outputs would be useful.