Isothermal vapour-liquid equilibrium (VLE) data were obtained for binary mixtures of diacetyl with toluene and cyclohexane at 318.15, 328.15 and 338.15 K with the aid of a dynamic VLE still [Phase Equilibria: Measurement and Computation, Taylor and Francis, Bristol, PA, 1998]. A computer was used to control the temperature and pressure in the still. The excess thermodynamic properties, molar excess Gibbs energy (G(E)), molar excess enthalpy or heat of mixing (H-E) and molar excess entropy (S-E) were calculated from the isothermal VLE data. Infinite dilution activity coefficients are also reported. The data were found to be thermodynamically consistent.