Detailed 31 P{ 1 H} NMR spectroscopic investigations provided eeper insight into the complex, multi-step mechanisms involved in the recently reported photocatalytic arylation of white phosphorus (P 4 ). Specifically,t hese studies have identified anumber of previously unrecognized side products, which arise from an unexpected non-innocent behavior of the commonly employed terminal reductant Et 3 N. The different rate of formation of these products explains discrepancies in the performance of the two most effective catalysts, [Ir(dtbbpy)(ppy) 2 ][PF 6 ]( dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) and 3DPAFIPN.I nspired by the observation of PH 3 as am inor intermediate,w eh ave developed the first catalytic procedure for the arylation of this key industrial compound. Similar to P 4 arylation, this method affords valuable triarylphosphines or tetraarylphosphonium salts depending on the steric profile of the aryl substituents.