Isothermal vapor-liquid equilibrium data for the binary systems of 1,1-difluoroethane (HFC-152a) + n-butane were measured at 273.15, 283.15, 293.15, 303.15, 313.15 and 323.15 K, respectively. The experiments were carried out using a circulation-type equilibrium apparatus to measure temperature, pressure, and the compositions of the liquid and vapor phases. The experimental data were correlated with the Peng-Robinson equation of state (EOS) using the Wong-Sandler mixing rules and the Camahan-Starling-De Santis equation of state (CSD EOS). Calculated results showed good agreement with experimental data. It was found that this system has very strong positive azeotropes for all the temperature ranges studied here. (C) 2007 Elsevier B.V. All rights reserved.