Data are presented on the phase composition, particle size distribution, EPR, and luminescence of ultrafine chromium-doped alumina powders prepared by detonation synthesis. The largest particles in the powders are shown to consist mainly of a Cr 2 O 3 solid solution in α -Al 2 O 3 . The luminescence spectrum of fine particles shows, in addition to lines characteristic of ruby, extra lines which are tentatively attributed to Cr 3+ ions incorporated substitutionally into θ -Al 2 O 3 .