The effect of phosphorus poisoning on the catalytic behavior of diesel oxidation catalysts was investigated over model and supplier monolith catalysts,i.e., Pd-Pt/Al2O3. The results of ICP and XPS from the vapor-phase poisoning over model catalysts suggested that the temperature of phosphorus poisoning affects both the overall content of phosphorus and the dispersion of phosphorus (i.e., inlet/outlet and surface/bulk). Phosphorus oxide (P2O5), metaphosphate (PO3-), and phosphate (PO43-) were identified in the poisoned model and supplier catalysts. The distribution of these species on poisoned model catalysts was highly dependent on the poisoning temperature,i.e., a higher temperature resulted in a higher concentration of PO43-. The outlets of the monoliths contained more PO(4)(3-)and less P(2)O(5)than the inlets. Both active sites and surface OH groups on model and supplier catalysts were contaminated upon phosphorus poisoning. It is found that PO(4)(3-)had a stronger influence on the active sites than P2O5. One significant finding in this study is that the vapor-phase phosphorus poisoning could be a practical and cost efficient approach to simulate an accelerated aging/poisoning process.