To further improve the yield and alleviate generation of hazardous side products, we set out to seek the chemical origin of overreduction. First, we investigated the stability of phosphite toward hydride. No conversion was observed when sodium phosphite (Na2HPO3) was ball-milled alone or treated with KH under typical reaction conditions, which rules out disproportionation or hydride attack on phosphite as an overreduction pathway. We then studied the reactions of polyphosphates with various equivalents of hydride (Table ). Treatment of pyrophosphate with half an equivalent of KH (H:P = 0.5:2) furnished phosphite in a good efficiency (74% based on hydride) without detected overreduction. However, reactions of sodium triphosphate (Na5P3O10) with substoichiometric KH (H:P = 1:3) resulted in a decreased yield of phosphite (60% based on hydride after 60 min, Table , entries 7 and 8) and observation of hypophosphite (H2PO2 -), an overreduction product not detected when higher hydride loadings were applied (Table ). Also identified in the reaction were isohypophosphate ([O2HP-OPO3] 3-) and other phosphorylated phosphites, from which hypophosphite is generated via nucleophilic substitution with hydride (Figure (ii),(v),(ix)). These results suggested that, counterintuitively, reducing the amount of hydride employed has led to an increasing tendency for overreduction. A plausible rationale for this is that phosphite, the desired product, can undergo phosphorylation under the reaction conditions (Figure (xi)) thereby activating it for a second reduction by hydride (Figure ). While direct hydride attack at the bridging P-group in triphosphate as a pathway toward phosphorylated phosphite cannot be ruled out at this point, such rise in overreduction is primarily attributed to phosphorylation of generated phosphite. Char- acterization of the reaction of Na2HPO3 with Na5P3O10 by 31 P NMR spectroscopy revealed that phosphite is susceptible to phosphorylation by triphosphate under typical reaction conditions to afford isohypophosphate which may give rise to overreduction. Because this process is in competition with phosphate reduction (Figure (vi),(x)), adding excess KH mitigates phosphorylation of phosphite by depleting reactive polyphosphates in the system and thus leads to diminished overreduction. In contrast, pyrophosphate was found to be inefficient at phosphite phosphorylation under these conditions (see SI), which explains the lack of overreduction in its reactions. The observed behavior can be attributed to the diminished leaving group capacity of orthophosphate relative to linear condensed phosphates.