The advantages of non-adiabatic fractionation over the conventional means of low temperature gas separation are outlined. Overflow packing, an oriented packing having the combination of good heat and mass transfer performance required for this application, is described and its mode of operation illustrated. A flooding velocity correlation for this packing has been developed. The theoretical relationships between heat and mass transfer during partial condensation of a binary mixture are reviewed and the design calculations for a partial condenser outlined. Individual liquid and vapour phase mass transfer coefficients in overflow packing have been measured and correlated, and the resulting data successfully used in predicting the composition changes occurring during the partial condensation of binary organic vapour mixtures. Some preliminary experiments on the partial condensation of air were made. Experimental measurements of heat and mass transfer during the partial evaporation of acetone/benzene mixtures have been made and analyzed.