The coefficients of viscosity and thermal conductivity of binary mixtures of xenon with krypton, argon, neon and helium have been determined experimentally over the full range of composition of each mixture at a pressure of 70 cm of mercury and at a temperature in the range 18.0 to 18.3°C. Values of the dimensionless quantity f = λ/ηcv have been obtained for each mixture. The experimental values are compared with theoretical values based on the Lennard-Jones (6-12) potential of interaction between molecules, using force constants derived from viscosity data. Agreement between the experimental and theoretical results is not complete. It is suggested that the discrepancies may be due principally to deficiencies in the empirical rule recommended in the literature for determining the force constant, 12/k, for interactions between unlike molecules.