Mixed crystals of potassium chloride and bromide show strong internal strain evidenced by cracking. The minimum of the melting point occurs between 63 and 65 mole % KBr. The microhardness between 25 and 65 mole % KBr is more than twice that of either component. The lattice constant changes linearly with composition. The macrodensity deviates by several percent, indicating a high concentration of defects. The intensity of coloration by electrons shows that defects are not mainly due to vacancies but presumably to larger aggregates of submicroscopic size or interstitials. The broadening of exciton bands, as measured by shifts of the ultraviolet absorption edge, is greatest at a 2:1 KCl&sngbnd;KBr ratio. This may indicate a certain short‐range order in KCl&sngbnd;KBr mixed crystals.