The selective oxidation of NH3 to N-2 in simulated biogas containing a large excess of CO and H-2 has been examined using zeolite catalysts. Of the materials examined zeolite Beta gave the highest N-2 yield (85% at 475-575 degrees C), while ZSM5 produced 75% at 575 degrees C, but HY was both less active and selective. In all cases N-2 is formed via an internal selective catalytic reduction between NOx (derived from the oxidation of NH3) and NH3 adsorbed on Bronsted sites of the zeolite.