The total vapour pressure of (CH3F + CH3Cl) was measured as a function of composition at temperatures of (159.01 and 182.33) K using the static method. These data were used to evaluate the molar excess Gibbs free energy G(m)(E) of the system at the same temperatures giving G(m)(E) (x = 0.50, T = 159.01 K) = 149 J . mol(-1) and G(m)(E) (x = 0.50, T = 182.33 K) = 175 J . mol(-1)'. No azeotrope was found at both temperatures. The molar excess enthalpy for the equimolar mixture estimated from the G(m)(E) values is low and slightly negative: H-m(E) (x = 0.50) = -29 J . mol(-1). (C) 2003 Elsevier Science Ltd. All rights reserved.