Process for preparing ammonium dichromate, comprising the steps of c) thermally decomposing an alkali metal ammonium chromate double salt, especially a sodium ammonium chromate double salt or hydrates thereof, at a temperature up to 200° C., especially of 75 to 190° C., to form ammonium dichromate and d) removing the ammonium dichromate from the decomposition product obtained after step c), by crystallization, characterized in that the alkali metal ammonium chromate double salt corresponds to the formula Mx(NH4)yCrO4 or hydrates thereof, in which M is Na or K, particular preference being given to Na, x is from 0.1 to 0.9, preferably from 0.4 to 0.7, y is from 1.1 to 1.9, preferably from 1.3 to 1.6, and the sum of x and y is 2.