In this Article, a novel permeation reduction coefficient (PRC) is defined and used to take into account the presence of both inhibition by CO and concentration polarization in hydrogen permeation through Pd-based membranes. The usefulness of this coefficient consists in the possibility of describing simply, but at the same time powerfully, the behavior of the membrane subject to the combined effect of inhibition and polarization. According to this approach, the effective permeance, which is generally unknown because it depends on these two phenomena, can be directly evaluated by multiplying the "clean" intrinsic membrane Sieverts permeance (measurable by simple pure hydrogen permeation tests) by a PRC function, that is, [effective permeance] = (1-PRC) [clean Sieverts permeance]. The values of PRC are evaluated by means of a complex model that takes into account the several elementary permeation steps, in which the inhibitory effect of CO is also considered as well as the concentration polarization. The membrane behavior is evaluated in terms of some "performance maps", where PRC and other two coefficients (concentration polarization coefficient (CPC) and inhibition coefficient (IC)) are reported as functions of several operating conditions (hydrogen molar fraction, CO partial pressure, and upstream total pressures). Therefore, these maps provide a useful tool to estimate directly the main design parameter (the overall permeance) in situations where complex transport and kinetic phenomena affect the membrane performances, allowing the membrane performance to be estimated much better and the separation equipment to be better designed.