Supercritical antisolvent process is a proven method for micronization and encapsulation of solute(s) dissolved in a solvent. When solution contains one solute, it is micronized to produce small particles; when solution contains two solutes, they either coprecipitate, or one of them encapsulates the other. Experiments with several systems for micronization and encapsulation have been described in the literature. In this work, a model has been developed for the process in which supercritical carbon dioxide flows countercurrently or cocurrently to an atomized solution. The model includes mass transport of the solute(s) and carbon dioxide, supersaturation, nucleation, and growth of particles of solute(s). Criteria for coprecipitation and encapsulation were identified, and on the basis of model predictions a new mechanism is proposed to explain the criteria. A solute encapsulates another solute if supersaturation of the former becomes high near the end of the process and it grows on the latter. If high supersaturation is attained during the process, they coprecipitate.