Silicon microreactors with Pt/Al2O3 thin-film wall catalyst were adopted for kinetic studies of CO preferential oxidation (PrOx). The activity of this catalyst was compared to that of other catalyst systems based on similar formulation. Internal and external mass-transport and heat-transport limitations of the microreactor were examined and comparisons were made to minimized packed-bed reactors (m-PBRs). We found that at lower temperatures (< 220 degrees C), the microreactor shows negligible mass- and heat-transport resistance, implying direct access to intrinsic kinetics. However, external mass transport begins to play a significant role in limiting overall reaction rates above 220 degrees C for PrOx. A microkinetic reaction model for PrOx was used for the study of reaction pathways and the analysis of surface intermediate species, which are difficult to study experimentally. Reaction mechanisms are discussed with these modeling results as a guide. Afterward, the results of a separate, nonisothermal reactor model using the finite-difference method are discussed to understand differences in performance of the microreactor and m-PBRs with respect to the CO conversion vs. temperature characteristic. As a result, we discovered that the temperature gradients in m-PBRs favor the reverse water-gas shift (r-WGS) reaction, thus causing a much narrower range of permissible operating temperatures compared to that of the microreactor. Accordingly, the extremely efficient heat removal of the microchannel/thin-film catalyst system eliminates temperature gradients and efficiently prevents the onset of the r-WGS reaction. (c) 2005 American Institute of Chemical Engineers.