The Diesel Oxidation Catalyst (DOC) is an important technology for the removal of CO and hydrocarbons (HC) from the exhaust of diesel engines, as well as for generating exotherms for active regeneration, and for producing NO₂ used by downstream components.This paper describes the development of a one-dimensional numerical model for a Pt-Pd DOC for use in designing aftertreatment systems. The model is based on kinetics developed from laboratory microreactor data. The model is a significant advance over previous DOC models we have developed. A much larger experimental matrix was used enabling the kinetics and inhibition effects to be much better defined. The experiments included rich conditions enabling the model to be used in NOX trap systems, where the exhaust becomes rich during regeneration. Reduction of NO₂ to NO by CO and HC has been included in the model. As well as converting NO₂ entering the DOC, this reaction prevents NO₂ formation while significant levels of CO and HC are present along the catalyst. The conversion of NO to NO₂ is observed to exhibit hysteresis when the temperature is ramped up and then down. This has been explained and successfully modeled in terms of the formation of an oxide layer on the surface of the catalyst, which is inactive for NO oxidation. Including both this effect and NO₂ reduction is important for obtaining a good prediction of the outlet NO₂ concentration, which in turn is crucial for the performance of any downstream model. The model has been successfully validated over both NEDC and FTP75 tests.Finally, it is demonstrated that this model, developed for an LDD catalyst, is readily adaptable to an HDD catalyst. The HDD version of the model has been extensively validated against engine test data for a range of tests (steady state light-off, transient light-off, HD-FTP, NRTC, WHTC).