We have combined Cu K-edge X-ray absorption spectroscopy with NMR spectroscopy (1H and 31P) to study the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction under operando conditions. A variety of novel, well-defined CuI iminophosphorane complexes were prepared. These ligands, based on the in situ Staudinger reduction when [Cu(PPh3)3Br] is employed, were found to be active catalysts in the CuAAC reaction. Here, we highlight recent advances in mechanistic understanding of the CuAAC reaction using spectroscopic and kinetic investigations under strict air-free and operando conditions. A mononuclear Cu triazolide intermediate is identified to be the resting state during catalysis; cyclization and protonation both have an effect on the rate of the reaction. A key finding of this study includes a novel group of highly modular CuI complexes that are active in the base-free CuAAC reaction.