Solubilities of p-hydroxybenzoic acid (p-HBA) and mixtures of the para and ortho isomers have been determined in supercritical carbon dioxide at 318 and 328 K for pressures in the range 101-203 bar. The solubility of p-HBA in carbon dioxide (p-HBA/CO2 binary system) is 2 orders of magnitude lower than the binary system solubility of o-HBA. However, measurements undertaken with an equilibrium chamber of mixed para and ortho isomers demonstrate a strong enhancement in the solubility of p-HBA. Furthermore, the enhancement was found to be independent of the composition of the equilibrium chamber. The binary and ternary (p-HBA/ o-HBA/CO2) solubility data were correlated using a modified form of the Peng-Robinson equation of state. Covolume-dependent mixing rules were also used with the Peng-Robinson model in an attempt to improve the correlation of solubility data. Conventional supercritical fluid (SCF) extraction of a mixture of the isomers is capable of producing an extract which is 99% pure o-HBA in one extraction step. The potential for separation of the isomers using retrograde crystallization appears limited because of the similar crossover pressure for both isomers.