The mechanism of photooxidation of triarylphosphines Ar3P under air was explored using a combination of a theoretical technique and an experimental technique. Computations based on density functional theory (DFT) were performed to simulate an IR spectrum of a possible intermediate (Ar3P+OO center dot), the absorption bands of which are in good agreement with the bands on the transient spectrum observed by laser flash photolysis-time-resolved infrared spectroscopy (LFP-TRIR). The radical cation Ar3P center dot+ generated upon LFP is likely to be trapped by O-2.