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The vapour pressures of benzene, cyclohexane, pentane, hexane and heptane, dissolved in mixtures of squalane and dinonyl phthalate, were measured at 30°C by a static method, and the compositions of the solutions found by means of a microbalance. Similar studies were carried out with the same solutes in polydimethylsiloxane (whose relative molecular mass was ascertained to be about 9 X 104) and in the polymer together with dinonyl phthalate. The partial miscibility of these last two was investigated at 30°C, Activity coefficients and, at infinite dilution, liquid/vapour partition coefficients of the solutes were calculated. Using a curve fitting technique allowing for errors in all the measurements, interaction parameters were determined on the basis of conventional solution theory. The relationship proposed by Purnell and Andrade between limiting partition coefficients for single solvents and those for the mixed solvents was found to be unsatisfactory for these systems.