The mechanism of ammonia synthesis and decomposition on transition metal surfaces has been analyzed using the BOC-MP (bond-order-conservation-Morse-potential) method. The analysis is based on calculations of the heats of chemisorption, Q, for all adsorbed species and activation barriers, ΔE∗, for all elementary reactions believed to be involved in the reaction N2 + H2 NH3 over Pt(111), Ru(001), Fe(110), Re(001). The relevant experimental values of Q and ΔE∗ agree well with the BOC-MP estimates. It is shown that along the periodic series Pt, Ru, Fe, Re, the dissociation activation barriers decrease but the recombination and desorption barriers increase. In particular, we find that on all the surfaces the largest activation barrier corresponds to the recombinative desorption 2Ns → N2. This step is projected to be the rate-determining process for ammonia decomposition, and Pt is projected to be the most efficient catalyst. For the dissociation N2 → 2Ns, we find that the activation barrier sharply increases in the order Re ⩽ Fe ⪡ Pt, which makes Pt surfaces incapable of catalyzing ammonia synthesis. These and other BOC-MP projections are in agreement with the results of mechanistic studies on Pt(111), Ru(001) and Fe(110).