Using data for heats of mixing h(E), temperature dependent parameters of the NRTL and UNIQUAC models have been estimated for 57 systems of binary mixtures. In order to test the models thoroughly, systems of various types, including associating and partially miscible systems, that have data for h(E) at more than one different isotherm are considered. Temperature dependences of the models are assumed as non-linear. This form of dependence is found to be more suitable for associating systems and for prediction of excess heat capacity C(p)E from differentiation of a correlation for h(E). No fixed values are assumed for the parameters of the models. The physical meanings and non-uniqueness of the parameters are discussed.