Producing chromium(III) oxide, comprises: (a) decomposing an alkali-metal-ammonium chromate double salt at 200-650[deg] C, preferably 250-55O[deg] C; (b) washing the decomposition product obtained in the step (a); and (c) calcining the product obtained in the step (b) at 700-1400[deg] C, preferably 800-1300[deg] C. The alkali metal ammonium chromate double salt comprises sodium-ammonium chromate double salt, which exhibits a molar ratio of ammonia to sodium of greater than 2. An independent claim is also included for producing the alkali-metal-ammonium chromate double salt of formula (M x(NH 4) yCrO 4) (I) or its hydrate, comprising adding ammonia at 55-95[deg] C to an aqueous solution of alkali dichromate of formula (M 2Cr 2O 7) (II). x : 0.1-0.9, preferably 0.4-0.7; and y : 1.1-1.9, preferably 1.3-1.6; or x+y : 2; and M : Na or K.