Disk-shaped dense membranes made of 25 mol% yttria doped bismuth oxide (BY25) in the fluorite-type fee phase structure (Bi2O3 phase) were fabricated by using press/sinter method from the citrate-derived BY25 powders. Oxygen permeation fluxes through these membranes were measured at different temperatures and oxygen partial pressures. The bulk-diffusion is the rate-limiting step for oxygen permeation through a 1.4-mm BY25 membrane with activation energy of about 70 kJ/mol and exponent of 0.25 for oxygen partial pressure dependency. C-2 (C2H4 + C2H6) selectivity and yield achieved for oxidative coupling of methane (OCM) in the BY25 membrane reactor were, respectively, in the range of 20-90% and 16-4%, At the same C-2 yield, the membrane reactor gives Ct selectivity of about 30% higher than the cofeed fixed-bed reactor packed with BY25 pellets under similar conditions. After OCM the BY25 membrane remained in good integrity but the reaction caused formation of some impurity phases on the membrane surface exposed to the methane stream.