Isobaric boiling point data for systems of 1,2-dichloroethane, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, and sec-butanol have been measured over the whole composition range at 300, 500 and 700 mmHg. The experimentalt-xdata have been used to predict Wilson parameters and equilibrium vapor compositions. The parameters are in turn used to predict activity coefficients and excess Gibbs free energies for these systems over the entire composition range. The values ofGEare positive in the systems at all pressures studied.