COx species evolved during transient exposure of Sm2O3 catalysts to very low pressure CH4/O-2 mixtures are monitored in real time by molecular beam mass spectrometry at 1100 K. The rise of m/z = 28 and 44 signals is significantly delayed relative to the onset of CH4 decay only on substoichiometric Sm2O3-y samples. This observation rules out CO and CO2 as primary products of methane heterogeneous oxidation and implies that their immediate precursor, most probably CH3(g), is slowly oxidized during the initial stages of Sm2O3-y surface reconstruction. Present results suggest the possibility of suppressing carbon oxides in the oxidative dimerization of methane by dynamic control of selective site populations.