The total vapour pressure and the density of liquid krypton + xenon mixtures have been measured at the triple-point of xenon, 161.38 K. Estimates have been made of the excess Gibbs energy of mixing GE and of the excess volume of mixing VE. For the equimolar mixture, GE½= 114.5 J mol–1 and VE½=–0.695 cm3 mol–1. These results have been used to test a representative selection of recent theories of solutions. If the geometric mean rule (Berthelot rule) for the energy of interaction of a pair of unlike molecules is assumed to hold, all theories except one predict a negative VE½, and a GE½ which is too small. The exception is the average potential model, which predicts a positive VE½ and a GE½ about twice too large. Relatively small departures from the Berthelot rule would bring the calculated GE into agreement with experiment, but the theories do not necessarily then give an improved estimate of VE.