A flue gas that contains small amounts of both HCN and NO.sub.x, produced, for example, by catalyst regeneration in the fluid catalytic cracking of a petroleum gas oil, is readily denitrified by the catalyzed reaction that proceeds approximately according to:HCN+NO.fwdarw.N.sub.2 (gas)+CO+CO.sub.2 +H.sub.2 OIf the molar ratio of HCN to NO in the flue gas is about 1.0, e.g. in the range of about 0.8 to 1.2, effective denitrification is achieved without first changing the composition of the flue gas by contacting it with catalyst under conversion conditions including elevated temperature. If the molar ratio of HCN to NO exceeds 1.2, the ratio may be adjusted to about 1.0 to 1.1 by thermal or catalytic oxidation in the presence of oxygen gas, followed by catalytic denitrification. If the molar ratio is less than about 0.8, the effective molar ratio is adjusted to about 1.0 to 1.1 by adding NH.sub.3 gas, followed by denitirification. In all three instances an excessive content of carbon monoxide in the flue gas may be corrected by passing the denitrified flue gas to a CO-boiler.Catalysts for the denitrification reaction are described and exemplified by crystalline zeolites of the ZSM-5 type, and by vanadium oxide supported on titania.