Readily available and bench-stable Cu(acac)2 (1) addresses many challenges in exploratory hydrophosphination catalysis. Mechanistic investigations were performed to answer questions that remain about the reactivity of 1, the role of light in the catalysis, and to provide direction for further study. A divergent Hammett plot indicates differing mechanisms based on electron density at the alkene substrate. A radical process was eliminated based on trapping reactions and in-situ EPR experiments. Isotopic labeling experiments, a zwitterionic trapping experiment, stoichiometric model reactions, and catalytic reactions using proxy intermediates indicate that both conjugate addition and insertion-based mechanistic pathways occur with this system, depending on the unsaturated substrate. Computational analysis indicates that the lowest energy transition is a ligand-to-metal charge transfer (LMCT) from the phosphido ligand where the LUMO has significant Cu–P antibonding character, suggesting that a weakened Cu–P bond accelerates insertion under photocatalytic conditions. This hypothesis explains the greater activity of 1 compared to copper-catalyzed hydrophosphination reports and appears to be a general phenomenon for copper(I) catalysts. These results have been leveraged to achieve heretofore unknown catalytic hydrophosphination reactivity, namely the diastereoselective hydrophosphination of a tri-substituted styrene substrate.