A one-dimensional model of NO oxidation and NO x storage on Pt/BaO/Al 2 O 3 monolith catalysts has been developed to predict evolution of the effluent gases as well as surface species during the storage portion of the NO x trap cycle. The model combines separate descriptions of oxidation and storage. The oxidation portion of the model is based on a kinetic mechanism and quantitative parameters developed in our laboratory for a Pt/ Al 2 O 3 catalyst. Two NO x storage models are proposed to account for the multiple time constants that control NO x storage. A two site in series model was developed to account for diffusion into the bulk of Ba particles, while a two site parallel model was proposed to account for Ba sites close to Pt and those far from Pt. It is shown that both models adequately account for the asymmetric nature of the NO x breakthrough curves and that current data cannot distinguish between the two models. It is also illustrated that external mass transfer plays a controlling role during early storage times and must be included in any model for the operating conditions used here. When possible, the model parameters were determined via independent experiments, while the remaining parameters were fit to the NO x uptake data using a nonlinear least squares method.