Catalytic properties of γ-Al2O3-supported complex vanadium and chromium oxides V2-xCrxO5-δ (0 < x ≤ 1.3), amorphous to X-rays, in the reduction of nitrogen oxides with ammonia were studied. Vanadium exists in these catalysts mostly in a pentavalent state and chromium exists as Cr3+ and Cr6+. As the content of chromium in the catalysts increases, the optimal temperature of the process decreases, and the degree of conversion of nitrogen oxides increases. © 1996 Plenum Publishing Corporation.