An exhaust gas treatment apparatus (20) for reducing the concentration of NO.sub.x, HC and CO in an exhaust gas stream (18) such as produced by a gas turbine engine (12) of a power generating station (10). The treatment apparatus includes a multifunction catalytic element (26) having an upstream reducing-only portion (28) and a downstream reducing-plus-oxidizing portion (30) that is located downstream of an ammonia injection apparatus (24). The selective catalytic reduction (SCR) of NO.sub.x is promoted in the upstream portion of the catalytic element by the injection of ammonia in excess of the stoichiometric concentration, with the resulting ammonia slip being oxidized in the downstream portion of the catalytic element. Any additional NO.sub.x generated by the oxidation of the ammonia is further reduced in the downstream portion before being passed to the atmosphere (22).