Wesley Salmon has advanced a new model of explanations of particular facts which requires that the explanans contain laws. The laws used in explanations (according to this model) are of the form P(A · C1, B) = p1 ... P(A · Cn, B) = pn. A condition imposed by Salmon on these laws is that the reference classes, i.e. A · C1 ... A · Cn, be homogenous with reference to the property B. A reference class A is homogenous with reference to a property B if every property which determines a place selection with reference to B within A is statistically irrelevant to B in A. It is argued here that the concept of homogeneity cannot in general be satisfied in scientific explanations and that even a weaker requirement, “epistemic homogeneity,” may be too strong.