Modeling of the water + acetic acid + CO2 system with the cubic plus association (CPA) equation of state (EoS) is presented involving correlation of the binary data and prediction of the ternary one, yielding very satisfactory results. The number of necessary association sites of water in CPA is investigated with an emphasis on the very difficult to model water + acetic acid binary. The results are compared with those of two SAFT-type models, the original and the simplified one (SSAFT), which use the same association term as CPA. The nonassociation contribution (physical part) in the CPA and the two SAFT models is examined, based on the results using the same association sites for water. It is concluded that the optimum number of sites for water depends on the other compounds present and, furthermore, that the nonassociation contribution of SAFT or SSAFTin spite of its complexitydoes not offer any advantage over the simple cubic EoS.