The present study investigates the decomposition of ammonia (NH3) over Ni and Ru catalysts using a kinetic model based on a reaction mechanism consisting of kinetically important elementary steps. Experimental results obtained over a wide range of reaction conditions were reproduced by the model. Recombinative N-2 desorption was the rate-limiting step on the Ru catalyst; whereas, the overall rate of NH3 decomposition was controlled by the NH3 dehydrogenation step on the Ni catalyst.