A useful method is introduced for the determination of H-2 partial pressure in a gas mixture to estimate conversions in selective dehydrogenation reactions by applying a palladium-silver (Pd-23 wt % Ag) membrane. The membrane is connected downstream of the dehydrogenation reactor and operated at the same temperature and pressure as the reactor. In this method, the tubular membrane outlet is closed, allowing H-2 to diffuse through the membrane wall until an equilibrium is established between the H-2 pressure and the H-2 partial pressure in a gas mixture inside and outside the membrane, respectively. Dehydrogenation conversions of methylcyclohexane to toluene in a packed-bed reactor calculated from the observed H-2 pressures inside the closed membrane were very close to those obtained by gas chromatographic analysis at 593-643 K, 15 bar, and liquid hourly space velocities of 1.9-5.2 h(-1). Additional measurements with gas mixtures of H-2-N-2 confirmed the reliability of the method.