An analysis of the conversion-temperature diagram for methane steam reforming is presented. A mathematical model of a membrane reactor (MR) containing a Pd-based membrane with ideal behavior was' developed to calculate the temperature and species profiles. The comparison of annular and tubular configurations showed different MR performance because of thermal effects. The equilibrium of the MR system was calculated, starting from the method of "reactors in series". The equilibrium curve of a MR was calculated as a function of the sweep factor (I), defined as the ratio of the sweep gas and methane feed Row rates. The existence region for a MR was defined as a function of I and is delimited by the adiabatic, isothermal, and equilibrium curves. Once I, the feed molar ratio, and the oven temperature were defined, the reaction paths of ah MRs fell within the existence region. This was observed for various values of reaction rates, membrane thicknesses, flow rates, overall heat transfer coefficients, and geometric parameters. The existence region of a MR is, therefore, an extension of that in a traditional reactor, achieving better performance with increased methane conversion.