The behaviour of NOx, storage catalysts for lean burn engine exhaust gas has been studied both experimentally and by mathematical modelling. It shows significantly different time scales between the storage and the regeneration process, which can neither be explained by different reaction rates nor by common transport resistances. Two differently detailed numerical simulation models have been developed, which are based on the assumption that diffusion into the storage particles is strongly affected by solid volume changes. The first model, a single particle model, assumes that the NOx storage is governed by the slow, diffusion-hindered formation of a dense nitrate layer in the storage particles which rapidly breaks up into carbonate during regeneration. The model equations and simulation results will be discussed. Based upon the model a simplified shrinking core type model was developed which can be used to simulate a complete monolith channel. With both models it is possible to describe and explain the chemical and physical processes during storage and regeneration, including the effect of incomplete regeneration. (C) 2004 Elsevier Ltd. All rights reserved.