Detailed P-31{H-1} NMR spectroscopic investigations provide deeper insight into the complex, multi-step mechanisms involved in the recently reported photocatalytic arylation of white phosphorus (P-4). Specifically, these studies have identified a number of previously unrecognized side products, which arise from an unexpected non-innocent behavior of the commonly employed terminal reductant Et3N. The different rate of formation of these products explains discrepancies in the performance of the two most effective catalysts, [Ir(dtbbpy)(ppy)(2)][PF6] (dtbbpy=4,4 '-di-tert-butyl-2,2 '-bipyridine) and 3DPAFIPN. Inspired by the observation of PH3 as a minor intermediate, we have 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.