A mathematical model for mass transfer between a droplet of organic solvent and a compressed antisolvent is presented. The model, which allows for two-way mass transfer both into the droplet and into the bulk antisolvent, is applicable to the supercritical antisolvent (SAS) method of particle formation. In this process, solute particles are formed by precipitation from an organic solution upon contacting with a supercritical fluid antisolvent which is miscible with the organic solvent, but immiscible with the solute. The mass transfer behavior of the droplets is thought to be a key factor affecting particle morphology. In this work, conditions that are supercritical with respect to the pure antisolvent, but subcritical with respect to the solvent-antisolvent mixture, are considered, meaning that two phases are always present. Radial profiles of composition and density as a function of time, both inside and outside the droplet, are generated by numerical solution of the governing equations. The radius of the droplet as a function of time is also determined. Calculations with toluene as the organic solvent and carbon dioxide as the compressed antisolvent show that the initial interfacial flux is always into the droplet, causing droplet swelling. Trends in the extent of droplet swelling as a function of operating conditions correlate with trends in the initial interfacial flux, which shows a non-monotonic pressure and temperature dependence and is especially sensitive near the critical locus of the mixture. At the mixture critical point, droplet lifetime diverges. Away from the critical point, the lifetime decreases as the pressure is increased, but shows a non-monotonic temperature dependence. However, the time required for a droplet to reach saturation at its center does not correlate with droplet lifetime, so that droplets with short lifetimes are not necessarily the fastest to reach saturation.