This study introduces a coupled reaction-diffusion model able to account for O-2 diffusion in the wall flow honeycomb particulate filter. The diffusion model is incorporated on a previously published single-channel model for the calculation of flow distribution, heat transfer, and soot oxidation reaction. A computational study is carried out aiming at evaluating the importance of O-2 diffusion effects as the function of operating conditions similar to those encountered in the real world. During controlled regeneration conditions, the effect of oxygen diffusion is minor and observable only at high soot loadings. On the other hand, the role of oxygen diffusion is shown to be critical in uncontrolled regeneration conditions, which are important for filter thermal failure. Traditional models neglecting diffusion effects are shown to strongly underpredict the maximum temperatures during uncontrolled regeneration with moderate to high soot loadings. (C) 2004 American Institute of Chemical Engineers.