Information on the interaction between different molecules can be derived from the temperature dependence of the compressibilities of gas mixtures. An alternative method is to measure the volume change on mixing two gases, but this method assumes knowledge of the virial coefficients of the pure gases. For both methods the required accuracy is so high that at present no reliable comparison can be made of the intermolecular energy of different molecules with the field of identical molecules. Several empirical rules have been proposed to estimate the interaction between different molecules from the forces acting between identical particles. It is shown that quantities calculated with the help of these rules are in fair agreement with experimental data.