A mathematical model for mass transfer between a droplet of organic solvent and a compressed antisolvent is presented for conditions such that the two phases are fully miscible. This work is complementary to our recent study, which described solvent-antisolvent mass transfer at mixture subcritical conditions. Both models are applicable to the supercritical antisolvent (SAS) method of particle formation. In this process, solute particles precipitate from an organic solution when sprayed into a compressed antisolvent continuum. The antisolvent 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. At mixture supercritical conditions, the antisolvent and solvent are completely miscible, and therefore no well-defined interface exists between the droplet and its environment. By defining an effective droplet radius based on the difference in density between the solvent-rich and antisolvent-rich regions, the extent of mass transfer can be tracked, and the effect of process parameters can be determined. Calculations with toluene (solvent) and carbon dioxide (antisolvent) show that droplets swell upon interdiffusion when the solvent is denser than the antisolvent. and shrink when the antisolvent is denser. Near the mixture critical locus, droplets exhibit greater swelling, longer lifetimes, and higher sensitivity to changes in temperature and pressure, compared to conditions removed from criticality. Examination of the time scales for mass transfer indicates that droplet lifetimes are significantly shorter for miscible conditions than for operation under bulk conditions corresponding to partial miscibility. As the mixture critical point is approached, from either the subcritical or the supercritical regime, droplet swelling decreases and lifetime diverges due to the vanishing of the diffusion coefficient at the critical point. (C) 2000 Elsevier Science B.V. All rights reserved.