The mathematical models for porous catalysts involving interparticle and intraparticle Knudsen diffusion with and without a first order irreversible reaction in a TAP reactor during a single pulse experiment were analyzed. If the ratio of the interparticle to the intraparticle transport characteristic times (gamma) is sufficiently large, the intraparticle concentration distribution follows an intraparticle pseudo-steady state (IPSS) condition. For a three-equal-zone reactor, the IPSS assumption is valid when gamma >= 12.5, corresponding to a macro-porous domain. For gamma <= 12.5, the validity of the EPSS assumption depends on the magnitude of the effectiveness factor. The expressions for the valid domain are proposed. The validity of the IPSS assumption for a thin-zone reactor is also discussed. Moment analysis shows that analytical expressions for the gas conversion are the same for the cases with and without application of the IPSS assumption. The conversion expressions for different shapes of catalyst pellets and different reactor configurations are reported. (c) 2005 Elsevier B.V. All rights reserved.