Semi-volatile secondary organic aerosols (SOA) comprise a major fraction of ambient particle pollutants. The partitioning of SOA in the atmosphere has commonly been assumed to be fast enough that it could be computed solely from thermodynamic equilibrium considerations e.g., using Raoult's Law. This simplifying assumption has been called into question by recent studies of single SOA particles evaporating in a zero-vapor concentration environment, which reported unexpectedly slow evaporation relative to atmospheric timescales. In this work we directly investigated the phase equilibration kinetics of systems of SOA particles under realistic atmospheric conditions. SOA was generated in an oxidation flow reactor (OFR) from engine exhaust or α-pinene and mixed with clean air in an atmospheric pressure smog chamber (32 °C) to induce evaporation. The evolution of the particle size distribution was monitored over time as the aerosol system returned to phase equilibrium under different particle concentrations (2.5 and 5 µg m<sup>−3</sup>) and humidity conditions (< 10 % and 60 %). We found that under typical ambient conditions, and independent of relative humidity and precursor origin (engine exhaust vs. α-pinene), SOA reestablished equilibrium with the vapor phase within minutes, and that the evolution of particle size was well-fit by a computational model treating the particle phase as well-mixed. The effective thermodynamic saturation concentration of the SOA was found to be in the range 0.02–0.11 µg m<sup>−3</sup> at 20 °C, assuming an enthalpy of vaporization of 150 kJ mol<sup>−1</sup>. The effective evaporation coefficient was found to be in the range 0.1–0.2 using a gas diffusion coefficient of 5 × 10<sup>−6</sup> m<sup>2</sup> s<sup>−1</sup>. Unlike previous single-particle studies, this data suggests that under most loading conditions, anthropogenic and biogenic SOA can rapidly attain phase equilibrium in the atmosphere and that their partitioning can be modeled assuming thermodynamic equilibrium.