Data for the mutual solubilities of fatty acid + water mixtures are scarce and so measurements for seven fatty acid (C-5-C-10, C-12) + water Systems were carried out. This new experimental data was successfully modelled with the cubic plus association EoS. Using data from C-6 to C-10 and the Elliot's cross-associating combining ride a correlation for the k(ij) binary interaction parameter, as a function of the acid chain length, is proposed. The mutual solubilities of water and fatty acids can be adequately described with average deviations inferior to 6% for the water rich phase and 30% for the acid rich phase. Furthermore, satisfactory predictions of solid-liquid equilibria of seven fatty acids (C-12-C-18) + water systems were achieved based only on the k(ij) correlation obtained from liquid-liquid equilibria data. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1604-1613, 2009