The thermodynamics of impure Heisenberg magnets have been studied previously within the random phase approximation (RPA) which neglects some important correlations between exchange‐coupled spins. To test the accuracy of the RPA and to allow more detailed comparison with experiment we have extended the rigorous Dyson low‐temperature boson formalism to impure systems. Spin‐wave interactions are treated in Hartree‐Fock approximation (equivalent to Dyson's first Born results for the pure system). We find only very small differences from the RPA impurity magnetization results, but there is substantial variation from RPA predictions of the temperature dependence of local mode energies.