The conjugate addition of an alcohol to a butynoate ester using an organophosphine catalyst was monitored using pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS), together with ³¹P and ¹H NMR spectroscopy. The combination of methods allowed examination of the reaction progress from the perspective of reactants and products (¹H NMR) and insights into behaviors of the reaction intermediates and formation of byproducts (³¹P NMR and MS). The resulting traces could be closely approximated through numerical modeling by appropriate selection of rate constants, and a sound understanding of the mechanism and the means by which oligomeric byproducts are formed allows a rational approach to experimental design.