We have studied the adiabatic and autothermal oxidative coupling of methane over Pt on alpha-Al2O3 monoliths at space velocities above 10(5) h(-1). The selectivity to Ct hydrocarbons (primarily acetylene) reaches a maximum of around 20% at low fuel to oxygen ratios, low dilution, and high space velocities. These conditions promote a large temperature gradient in the monolith, with an exit temperature of nearly 1500 degrees C and an entrance temperature of less than 200 degrees C. This temperature gradient appears to be the driving force far C-2 hydrocarbon formation under these conditions. Both homogeneous and heterogeneous reactions may be involved in producing coupling products, and a combustion model predicts C-2 selectivities similar to those observed.