The effect of substitutional impurities on the low temperature spontaneous magnetization of cubic ferromagnetic crystals is investigated in the spin wave formalism with the aid of techniques developed previously for the study of defect problems in lattice dynamics. The impurities considered have an exchange integral with their nearest neighbors different from that between neighbors in the pure crystal. The results of these calculations show that the Bloch T32 law is still obeyed, but that the coefficient of the T32 term is a function of the impurity concentration whose form is determined in the limit of low impurity content.