Many studies have demonstrated an inhibition of hydrogen permeance through a Pd or Pd-alloy membrane due to competitive adsorption of CO, ammonia and other species. Several corrections to Sieverts' law were suggested to account for these effects, based on a Langmuir isotherm, but they all suffer from certain drawbacks as detailed below. We derive a new correction, based on the kinetic balances on the adsorbed layer, and on the underlying dissolved H layer from which the diffusing H atoms originate. The new correction is physically reasonable and it predicts that the permeation inhibition declines with increasing membrane size, in agreement with several suggestions in the literature. We test the new expressions against published experimental data of inhibition due to CO co-adsorption, using energetic parameters that were either estimated by DFT studies or derived from experimental data. A fair agreement is obtained, but it depends on assumptions regarding entropy effects. We also derived an approximation and adjusted its parameters. Finally we comment on the observation of inhibition due to H2S and ammonia, which represent more complex cases.