The reaction of sodium pyrophosphate (Na4P2O7) with one equivalent of KH (H:P = 1:2) furnished phosphite in 37% yield within 60 min of mixing. While elongating the reaction time at the same hydride loading did not improve the yield, an increased yield of phosphite (44%), which corresponds to 88% of reactive P-units, was obtained when a higher hydride loading (H:P = 1.5:2) was used followed by 90 min of mixing. A similar trend was observed in the reduction of sodium triphosphate (Na5P3O10) by KH. Mixing Na5P3O10 with two equivalents of KH (H:P = 2:3) yielded phosphite in 44% yield within 60 min, while an increased yield of phosphite (58%), which corresponds to 87% of reactive P-units, was obtained at a higher hydride loading (H:P = 1:1) after 90 min. Only a very low degree of overreduction was observed. Potassium phosphate salts react with KH in a similar fashion to sodium salts, but more slowly. Sodium fluorophosphate (Na2PO3F) engendered a moderate yield (47%) and a notable degree of overreduction. Commercial "sodium hexametaphosphate" or Graham's salt, which is a mixture of cyclic and linear phosphates (Figure ), afforded a low yield of 29%. Lastly, orthophosphate gave rise to no phosphite, confirming that the presence of phosphoanhydride bonds is key to such reduction reactivity by loss of phosphate as a leaving group. The reactivity of phosphoanhydride bonds predominates biological phosphorylation processes, which involve removal of a terminal PO3 -group, such as in processes that consume ATP and produce ADP. Mechanochemical reduction of Na4P2O7 by KH followed by treatment with deuterium oxide (D2O) resulted in no detected deuterium incorporation in the formed phosphite as determined by P NMR spectroscopy, indicating that KH was the only source of hydrogen atom. This result confirmed phosphorylation of hydride as the exclusive mechanism of phosphite P-H bond formation in the current system, ruling out a mechanism of KH reduction of phosphates to yield Hfree low-oxidation-state phosphorus intermediates followed by protonation during aqueous workup. c Defined as recovery efficiency of total P content by aqueous extraction, determined by 31 P NMR spectroscopy. d 90 min. e For polyphosphates PnO3n+1 (n+2)-, bracketed value of yield is based on (n -1) reactive P-units according to Eq. 4 (two for triphosphate, one for pyrophosphate).