The gas antisolvent crystallization (GASC) process using dense carbon dioxide (CO2) as antisolvent is particularly useful for purification and micronization of thermo-labile bioactive solid substances. Conventionally, the GASC process is characterized by the relative total volume expansion or the relative molar volume expansion of the solution. A new criterion is proposed in this work in terms of the relative partial molar volume reduction (RPMVR) of the solvent for selection of the solvent and the optimum process condition for the GASC process, as it directly gives a measure of the fraction of the dissolved solute crystallized. The solute solubility is proportional to the partial molar volume of the solvent, (v) over bar (2) which drastically decreases at a high CO2 dissolution. This is attributed to clustering of CO2 molecules around the solvent molecules causing the loss of solvent power. This results in the desired antisolvent effect for lowering the solute solubility. (v) over bar (2) has been calculated for a large number of solvent-CO2 liquid mixtures using the Peng-Robinson equation of state. It has been observed that (v) over bar (2) drastically reduces at a high value of x(1), irrespective of the fact whether the solvent density is higher or lower than that of the CO2. The solute solubility has been predicted from its value at the ambient pressure and the ratio of the partial molar volumes of the solvent with and without CO2 dissolved in it. The predicted solubility of beta-carotene in ethyl acetate with variation of x(1) at 298 K has been found to compare well with the experimentally observed trend of the GASC process. (C) 2002 Elsevier Science B.V. All rights reserved.