A recently reported [J. Chem. Phys. 2004, 120, 6648] two-state association model (TSAM) is reformulated to obtain an expression for the residual Gibbs free energy of pure self-associating liquids. From the proposed partition function, where association is considered a perturbation, a vapor pressure correlation was obtained, with a clear and separable nonspecific contribution. The new formulation does not change the reported three-or four-parameter equation for calculating residual heat capacities, their values being determined solely from experimental heat capacity data. Two new parameters are necessary to describe the nonspecific reference fluid Gibbs free energy. The resulting vapor pressure equation was fitted to experimental data for pure 1-alcohols, 1-thiols, 1-amines, and carboxylic acids with very good agreement, up to the critical point.