In this work, regular solution theory was applied to study the solubility of solids in a supercritical fluid (SCF) with and without cosolvent, and a new model for binary and ternary systems was proposed. The activity coefficient can be obtained from the model and the solubility can then be calculated easily. For a binary system there are two adjustable parameters and for a ternary system, four adjustable parameters; the parameters are related to the interactions between molecules in solution. The proposed model was compared with the HSVDW1 and HSVDW2 models. The calculated results show that the proposed model is more accurate, and the AAD for the three models is 4.5%, 7.9% and 18.5%, respectively. The model was further used to correlate the solubility data of 2-naphthol in SC CO₂ with and without cosolvent measured by us before, and the overall AAD is 3.23%.