Separations of coproduct valeric acid from mixed dibasic acids is essential for a new process of producing adipic acid for nylon production. The batch fractionation of valeric acid from mixtures of valeric acid and dibasic acids consisting of methyl glutaric acid and ethyl succinic acid was carried Out with supercritical carbon dioxide using a continuous flow saturation apparatus. Phase equilibria of valeric acid and mixtures of valeric acid with methyl glutaric acid with supercritical carbon dioxide were measured in the range 35-65 degrees C and 68-140 bar. Apparent equilibrium of a mixture of valeric acid, methyl glutaric acid, and ethyl succinic acid with carbon dioxide is also reported. A high-pressure view-cell allowed visual observation of the number of phases present. Results indicate that highly selective removal of valeric acid is possible from the mixtures of dibasic acids using supercritical carbon dioxide at 50 degrees C and 90 bar, with loadings of valeric acid close to 0.1 mol %. Solubility of valeric acid in the fluid phase was increased to almost 6.5 mol % at 50 degrees C and 138 bar but with a decrease in the selectivity for valeric acid. The data obtained were correlated using the Peng-Robinson equation with conventional quadratic mixing rules.