A microkinetic model for oxidative coupling of methane (OCM) has been developed that comprises a reaction network of 39 gas-phase and 26 catalytic elementary steps. It has been implemented in a heterogeneous reactor model that explicitly accounts for the interactions between gas phase and surface species. Concentration gradients arising from mass transport limitations are found to develop inside the catalyst pellet for all intermediates (i.e., surface and gas-phase) even under an intrinsic kinetics regime for the molecules and clearly affect the C2 selectivity. Special attention has been devoted to the reduction of the number of adjustable parameters in the model and the a priori determination of thermodynamic as well as kinetic parameters. A contribution analysis is conducted in order to elucidate the complex reaction pathways in OCM that lead to the desired products. Apart from the methyl radicals that couple to an extent of almost 70% in the void space between the pellets, the catalyst pellet accounts for the majority of molecules and radicals conversion, which are produced on the surface and further interact either in the catalyst pores or with other surface species. Almost 95% of CH4 consumption and more than half of the C2H6 production take place inside the catalyst pellet. A similar analysis is applied to understand the effect of various textural properties of catalysts on the performance of OCM, for example, increasing the catalyst porosity is found beneficial for the C2 yield, as long as a sufficient CH4 activation takes place, so that the coupling pathway is promoted over the heterogeneous oxidation of CH3·.