Perturbation theory is applied to a mixture of hard spheres and square-well molecules. If the higher order perturbation terms are adequately accounted for, excellent agreement with machine simulation results is obtained. In particular, good values for the heat capacity are obtained. This is in contradiction with Alder's conjecture that the hard-sphere reference fluid is unable to deal with concentration fluctuations. In addition, an approximation to the higher order terms in perturbation theory which appears useful at low densities is suggested. Finally, the theory of random mixtures (RM) and the van der Waals (vdW) theory of mixtures, which are identical for this system, are considered. The estimates of the higher order perturbation terms given by the RM and vdW theories are seriously in error for this system. Copyright © 1974 American Institute of Physics.