Viscosity measurements, along with the Stokes–Einstein relation, have been used to estimate the diffusion rates of organics within SOA particles. To test the Stokes–Einstein relation, we measured the diffusion coefficients of three fluorescent organic dyes within sucrose–water solutions with varying water activity. The diffusion coefficients were measured using fluorescence recovery after photobleaching. The results should be useful for predicting the diffusion of organics with SOA particles.