The CPA (Cubic-Plus-Association) equation of state has been extended to modeling of organic acids. We will focus in this work on formic, acetic, and propanoic acids due to their importance to the chemical and petrochemical industries. Organic acids, unlike many other associating compounds, have a strong tendency to dimerise in the vapor phase at normal condition resulting in strong non-ideal behavior, even at low pressures. Pure compound parameters have been determined from vapor pressure and liquid density data for the three acids. Among the three tested association schemes (one-site, two-site, and four-site), only the one-site association scheme describes satisfactorily the association in both the gas and the liquid phase. Second virial coefficients axe predicted well with the proposed one-site model. Excellent binary VLE and acceptable LLE correlations have been obtained for acid + aliphatic hydrocarbons employing a low value of a binary interaction parameter. (c) 2004 Elsevier B.V. All rights reserved.