This paper presents a two-part study on the effect of Pt:Pd ratio (at a constant total Pt+Pd loading of 120 g ft−3) on the catalytic performance of a Diesel Oxidation Catalyst (DOC) intended for light-duty applications, covering ratios across the full range from 100% Pd to 100% Pt. (Work on a heavy-duty DOC is presented in SAE 2015-01-1052).The first part of this paper presents a reactor study on the effect of Pt:Pd ratio on the catalytic activity of key reactions occurring individually over the DOC, including the oxidation of CO, C3H6, n-C10H22, CH4 and NO. For some reactions, activity increases continuously with Pt content (oxidation of n-C10H22 and NO); in contrast the activity for CH4 oxidation increases with decreasing Pt content (increasing Pd content), while CO and C3H6 oxidation exhibit more complicated dependencies.The second part presents the development of a one-dimensional model capable of predicting the effect of Pt:Pd ratio on DOC performance. The model was based on a previous DOC model with selected parameters being varied to account for the change in Pt:Pd ratio. The variation in each kinetic parameter with ratio was described by a function to facilitate interpolation to any Pt:Pd ratio. For pragmatic reasons (the complexity of Pt-Pd catalysts), the model was developed directly from NEDC test data with kinetic parameters being optimised to achieve best fit to measured data for each ratio. The model gave a good prediction of post-catalyst CO, THC and NO2 emissions over the NEDC across the full range of Pt:Pd ratios.