We have developed membrane reactors to dehydrogenate cyclohexane, using amorphous silica membranes with controlled pore sizes, and successfully operated them under pressures ranging from I to 8 atm. The membranes were prepared by chemical vapor deposition (CVD) using tetramethoxysilane (TMOS). or dimethoxydiphenylsilane (DMDPS). Both membranes had hydrogen-permselective properties. At 473-573 K, the TMOS-derived membranes showed hydrogen permeances on the order of 10(-8) mol m(-2) s(-1) Pa-1, whereas the DMDPS-derived membranes showed permeances on the order of 10(-6) Mol m(-1) s(-1) Pa-1. Both membrane reactors produced hydrogen with conversions greater than equilibrium, and the DMDPS-derived membrane achieved higher conversion than the TMOS-derived membrane because of the greater hydrogen extraction effect of the DMDPS-derived membrane. In addition, it should be noted that this is the first report of successful operation of membrane reactors with highly hydrogen-permselective silica membranes under pressure.