The modified group contribution PC-SAFT has been extended in this paper to predict phase equilibrium of carboxylic acids and their mixtures. This work will focus on acetic and other linear organic acids due to their importance to the chemical industries. The new association energy parameter of acetic acid and carboxylic acids shows better agreement with the experimental results at approximately 300 K compared to all previously published SAFT-type parameters. New functional group parameters for carboxylic acids were regressed on selected pure compound data, and the binary interaction parameter (k(ij)) of binary mixtures was fitted using their liquid-liquid phase equilibrium data. The association schema and dipolar interaction of acetic acid were investigated using PC-SAFT EoS, with an emphasis on the difficulties in predicting liquid-liquid equilibrium of acetic acid and alkanes mixtures. The computation results were improved clearly when acetic acid was considered as dipolar and 1A associative molecule. The polar parameters obtained for acetic acid were then transferred to other linear carboxylic acids to determine the COOH group parameters. Good vapor-liquid equilibrium prediction results were obtained for acid + acid, acid + hydrocarbon mixtures, without using binary interaction parameter, k(ij) = 0.