A process for preparing a catalyst provided in the form of a metal oxide catalyst having at least one element selected from Mo, Te, Nb, V, Cr, Dy, Ga, Sb, Ni, Co, Pt and Ce. The catalyst is subjected to an aftertreatment to increase the proportion of the M1 phase, by contacting the catalyst with steam at a pressure below 100 bar or by contacting the catalyst with oxygen to obtain an aftertreated catalyst. The aftertreated catalyst may be used for oxidative dehydrogenation processes.