Competitive adsorption and reactant inhibition are expected to couple with diffusion resistance to form complex patterns within porous adsorbents and catalysts, especially under dynamic conditions. CO oxidation over Pt/Al2O3, which exhibits these surface processes, was studied in this work. A miniaturized single-pellet reactor was used to continuously and directly measure gas concentration gradients within a porous pellet as-feed composition was varied. Responses of nonadsorbing species were analyzed in order to determine diffusion coefficients. Single-species responses provided starting estimates of CO adsorption parameters. Reaction experiments were performed at 398-448 K under maximum CO and O-2 pressures of 16 and 110 Pa, respectively. An elementary-step diffusion-reaction model was used to simulate experimental results. A bimodal distribution of active sites was proposed in order to explain the transient responses. Analysis showed that the bimodal distribution could not have been detected by steady-state experiments. The intrapellet spatiotemporal patterns predicted by the model, and confirmed by comparison to measurements of concentration gradients across the pellet, provide understanding of the dynamic coupling between diffusion, adsorption, and reaction within the pellet. Methods are discussed to reduce computational requirements, including assumption of quasi-equilibrium adsorption, and reduction of the surface-to-gas capacity ratio.