Understanding the real aging of Diesel oxidation catalysts (DOC) is key to predict their useful lifetime. This work comprises a detailed study of a commercial DOC in its unused state and after use in vehicle for 288,000 km. The activity of the samples was correlated to a range of characterisation results obtained by microscopy, energydispersive X-ray spectroscopy, 31P nuclear magnetic resonance and infrared spectroscopy. While noble metal sintering had a minor role, the deactivation of aged DOC was predominantly driven by chemical poisoning. The distribution of the poisons in the washcoat revealed a preferential accumulation of P over S; additionally the poison concentration trends point P to be particularly detrimental for NO oxidation. The fundamentals of poisoning were studied by modifying the unused catalyst with P. The oxidation state of the active metals was affected by the presence of 2.5 wt.% P providing further insight into the deactivating role of this poison.