Solid solution hardening has been reviewed with respect to existing experimental data. It is shown with the aid of new experimental data on two inorganic systems (NiO-CoO and CaF(2)-SrF(2)) that the large increase in hardness of intermediate compositions correlates qualitatively and can also be quantitatively justified by the use of selected values of the parameters in a previously developed atomistic hardness formula. The key parameters are the variations in the unit cell dimensions, the eigenfrequency, and anharmonic factor of the structure.