The effect of intra-phase diffusion for channel-flow oxidation reactors with washcoats ranging in thickness from 10 to 80 mum is explored in combination with a detailed surface chemistry for low-temperature (less than or equal to200degreesC) H-2 oxidation over supported Pd/PdOx catalysts. A numerical model of a porous catalyst washcoat is developed to assess how local conditions influence catalyst effectiveness when considering a detailed multi-step surface mechanism, and this washcoat model is integrated into a channel flow reactor model to assess if and when effectiveness correlations may apply for channel flow reactors. The Pd-H-2-O-2 surface chemistry mechanism, which is validated against experimental measurements in an annular flow reactor, implements thermodynamically consistent interaction potentials of surface species and predicts non-linear behavior of conversion with respect to H-2 concentrations at the low equivalence ratios of the current study, particularly at lower temperatures (< 150degreesC) where surface chemistry dominates overall reaction rates. The catalytic washcoat model further indicates that conversion and similarly catalyst effectiveness are strongly dependent upon the site fractions of vacancies available for H-2 adsorption, which vary strongly with flow conditions and at higher conversions with depth in the porous washcoat. This leads to difficulty in developing simple models for catalyst washcoat effectiveness based upon any parameter such as a Thiele modulus. Furthermore the results suggest that care should be taken in interpreting kinetic data for oxidation reactions even when relatively thin washcoats are employed for reaction rate studies. (C) 2004 Elsevier Ltd. All rights reserved.