A microkinetic model was developed and applied to simulate an extensive experimental dataset on methane steam reforming. The data set consisted of 537 data points, and was collected over a Ni/NiAl2O4 catalyst at 843, 858, and 873 K in a wide steam-to-carbon ratio range between 0.2 and 7.1. A microkinetic engine modeling tool was applied for the construction and parameter estimation of an elementary step network consisting of 12 reversible reactions. Specific reaction pathways were implemented for the formation of CO and CO2, respectively. The model qualitatively if not quantitatively explains inverse and normal isotope effects experimentally observed at low and high S/C ratios, respectively, and is thermodynamically consistent. A combination of the approach to partial equilibrium (i.e. whether the elementary step proceeds forward or backward) and net rate (i.e. the difference between forward and backward reaction rate) provides novel insight into the oxidation mechanism of CHx and CHxO surface species.