The influence of the steady-state partial pressure of oxygen and of non-steady state operation on product distribution in the oxidative coupling of methane over a CaO/CeO2 catalyst was simulated on the basis of a kinetic model which takes into account surface processes. The simulations indicated that an optimal ratio of CH4-to-O-2 partial pressure leads to a maximum yield of ethane while the ethane selectivity decreases with increasing oxygen partial pressure. Non-steady-state operation could probably result in a significant increase of ethane yield if the operation would be started with an oxidized surface which is covered only by mono-atomic but no molecularly adsorbed oxygen species : when reaching steady-state operation ethane selectivity decreases by about 10%. If a reduced catalyst would be used for the start of the non-steady-state process the C-2 yield would not exceed the steady-state value while a maximum of C-2 selectivity is predicted which exceeds the steady-state selectivity of ethane by 20%. However, this maximum is related to very low methane conversion.