The chromate (VI) ion HCrO4− at pH 2—4 was not appreciablly reduced by the phosphinate ion PH2O2− in the absence of molecular oxygen (O2), but was reduced in the presence of O2, forming peroxodiphosphate ions (denoted by PDP and meaning all forms of P2O84−, HP2O83− etc.) in much higher concentrations than those of the added chromate(VI). Both the rates of the chromate decay and the PDP formation increased with decreasing pH. When the chromate(VI) ion disappeared completely, PDP formation stopped and its concentration remained constant. However, such PDP once formed in the presence of O2 began to decompose after the solution was saturated with N2 gas; its decomposition ceased again when the solution was again saturated with air. Such behaviors are discussed while presenting the reaction mechanisms.