Mixed solution-grown two-phase NaCl-KCl crystals are found to be structurally different from the melt-grown single-phase (NaK)Cl crystals. In the mixed crystal, quench hardening increases steadily with temperature which contrasts with the appearance of the plateau at 250-450 degrees C in pure NaCl and pure KCl crystals. These studies suggest that more than one mechanism is predominant in the mixed crystal. It is found that the interactions between the interphase diffused ions acting as impurities in the other phase and the dislocations, as well as the misfit strains introduced by these ions, contribute mainly to the hardening mechanism. Etching studies carried out also supported this finding.