Oxidative coupling of methane (OCM) on oxygen-semipermeable delta-phase 25 mol% Y2O3/Bi2O3 (BY25) in a reducing (methane) atmosphere was studied by measuring transient catalytic properties and weight change of the ceramics when exposed to a methane stream at high temperatures (>800 degrees C). Compared to the results obtained in a fixed-bed reactor operated in the cofeed mode, BY25 ceramics in a pellet form exhibit extremely high C-2 selectivity (94%) and appreciable reactivity (C-2 formation rate of 0.7 mu mol/g . s for OCM in a highly reducing methane atmosphere. Experimental results are also reported to show the effects of temperature, flow rate, methane exposure time, helium purge time, and helium to methane ratio on catalytic properties of BY25 pellets in a methane atmosphere. The results indicate that BY25 ceramic membrane materials possess desired catalytic properties for OCM membrane reactor applications.