The work described in this Thesis is concerned with the application of vapour sorption techniques employing vacuum microbalances to the study of liquid mixtures. A conventional quartz beam microbalance was used to measure the absorption of a range of volatile organic solutes by poly(dimethyl siloxane), PDMS, to determine infinite dilution activity coefficients and interaction parameters which agreed well with results from a joint gas-liquid chromatographic investigation. A slight dependence of these properties on polymer molecular weight and on the polymer to solid support ratio was detected. Results from the absorption of hexane by mixtures of PDMS with squalane or DNP were used to calculate inter-solvent interaction parameters and these were shown to give reasonable predictions of the miscibility limits of the mixtures. A recently developed magnetic suspension vacuum microbalance was shown to give accurate results for polymer solutions over a wide concentration range using PDMS as an example. Meaningful values of the partial molar enthalpy of mixing were measured for benzene and hexane with PDMS. A number of solution theories were applied to these results but they did not predict satisfactory solution properties in the high polymer concentration region. Previous work on the retention behaviour of mixtures of squalane and DNP was extended using three polar absorbates: chloroform, dichloromethane and ethyl acetate. It was found that predictions of the partition coefficients using the Purnell-Andrade equation were in error by up to 10% while those using the Tiley-Perry relationship agreed with experiment to within, on average, 3-4%. Variation of absorption with absorbent liquid loading in the (ix) benzene-PDMS and ethyl acetate-squalane or DNP systems was examined and the results suggested that adsorption occurred at the gas-liquid interface as well as at the surface of the solid support.