The underlying work is part of an extensive study to develop appropriate thermodynamic criteria for an optimum gas-antisolvent (GAS) process. It turned out that the ＇classical＇ definition of volume expansion is inappropriate to define these criteria. An alternative definition of this property has been introduced in this work. This study was carried out for a number of binary and ternary systems of which the experimental data were taken from literature. The Peng-Robinson equation of state was used to describe the fluid phases, whereas for the solid phase an independent relationship was applied. It will be shown that the definition for liquid-phase volume expansion as proposed in this work is useful for the selection of the optimum pressure for the GAS process. (C) 2000 Elsevier Science B.V. All rights reserved.