Detailed understanding of the poisoning mechanism in Diesel oxidation catalysts (DOCs) is the key to prevent long term catalyst deterioration in vehicles. The present work studies artificial phosphorus (P) poisoning methods applied to a commercial DOC. Cores of the DOC monolith were exposed to increasing amounts of P precursor by impregnation and by spraying. While impregnation leads to a homogeneous poison distribution in the catalytic layers, elemental analysis revealed that the spraying results in radial and axial P gradients that mimic the distribution of this poison previously observed in vehicle aged DOCs. The activity tests under simulated Diesel exhaust evidenced that P deposition by spraying was more detrimental than by impregnation and caused a more severe decrease in NO oxidation performance. Sequential treatments comprising exposure to P, thermal treatment and exposure to sulphur (S) interleaved by activity tests were performed to gain deeper insight into the catalyst deactivation mechanism.