We recently reported the synthesis of five-membered phosphorus/nitrogen-containing heterocycles 1 and 2 (Scheme 1) via the cycloadddition reactions of azophosphines (MesN2PR2; Mes = mesityl; R = i Pr, t Bu) with diethyl acetylenedicarboxylate ((C(CO2Et))2). Structurally analogous species were also reported by the Cummins group. Addition of water to a THF solution of heterocycle 1 resulted in clean conversion to a new compound, pyrazolyl phosphine oxide 3 (Scheme 1), with a 31 P NMR chemical shift of 62.3 ppm. This reaction features loss of the initial phosphorus/nitrogen-based heterocycle, and formation of a hydroxypyrazole core with an exocyclic phosphine oxide moiety. Overall, the transformation of 1 to 3 corresponds to addition of H2O and loss of EtOH, with the phosphorus centre remaining in a P V oxidation state. The reaction can proceed slowly if 1 is simply exposed to air (full conversion within four days), but can be carried out in under four hours if a large excess (100 equiv.) of H2O is added directly to the solution of 1. Although 1 reacts readily with H2O to form 3, it was not observed to react with other O-H or N-H bonds; the reactions of 1 with NH3, and various anilines and alcohols yielded no reaction when monitored by 31 P{ 1 H} NMR spectroscopy, even with heating to 60 °C over two days (see ESI for details). Interestingly, the bulkier analogue 2 (R = t Bu) showed no reactivity towards H2O by 31 P{ 1 H} NMR spectroscopy, even with a significant excess of H2O (100 equiv.) and heating to reflux in various solvents (see ESI for details).