A differential method is described for measuring the second virial coefficient of a gas from approximately the normal boiling-point of the substance to room temperature, given the value of the virial coefficient at one temperature. The imperfection of the gas is compared directly with that of a reference gas (helium) for which the second virial coefficient is known over the whole temperature range. Measurements have been made on argon, krypton, and methane, and on a sample, approximately equimolar, of each of the three binary mixtures which can be formed from these gases. The results for the pure gases have been used to demonstrate the superiority of the 18-6 form of the Lennard-Jones potential over the 12-6 form in accounting for low-temperature virial coefficients, while the figures obtained for the mixtures have been applied to examine the validity of three combining rules for estimating the intermolecular energy parameters for a pair of unlike molecules from the corresponding parameters for the two pairs of identical molecules.