The solution strengthening of sodium and potassium halide crystals by dissolved alkaline earth atoms is accounted for quantitatively. Strengthening is associated with the change in electrostatic energy that occurs when a divalent ion‐vacancy complex is sheared by a dislocation that passes through it. The model yields a hardening coefficient of 11 × 109dyn/cm2compared with the experimental value of 9.2 × 109dyn/cm2.