The mechanism and kinetics of NH3 decomposition reaction mechanism has been a subject of interest for decades, although it is attracting renewed interest in connection with hydrogen generation for fuel cells. This system represents an example of a most peculiar nonlinear catalytic reaction sequence for which, even though the mechanistic steps are well-accepted and their DFT energetics widely studied, the microkinetics have not yet been fully investigated. In fact, there is no agreement even as to which step(s) are rate-limiting. Employing our Reaction Route (RR) Graph approach and the corresponding electrical analogy, we investigate in detail the steady-state microkinetics of the seven-step NH3 decomposition reaction mechanism. Based on an Ohm's law representation of kinetics, we develop an accurate quasi steady-state (QSS) rate expression for the complete sequence, not possible with the usual QSS approach, which provides only numerical results for the nonlinear sequence. Further, it allows the rate-limiting step(s) to be identified in a rigorous manner, based simply on a comparison of the step resistances. Combining this with identification of the most abundant reactive intermediate (MARI), the mechanism and the rate expressions are pruned to provide a simplified rate law that is found to be in complete agreement with numerical QSS analysis as well as with our experimental data on Fe without any fitted parameters. (C) 2011 Elsevier Ltd. All rights reserved.