The zeroth-order reaction of the hydrolysis of sodium polyphosphate in water at pH = 0 can be explained assuming the formation of a pentacovalent terminal phosphorus intermediate, A terminal unit is activated for hydrolysis by protonation of the double-bond oxygen on that unit followed by a nucleophilic attack of water. Proton transfer from an OH group to a P-O-P bond breaks the bond, thereby shortening the phosphate polymer. A mathematical equation is developed to explain the reaction order of the hydrolysis.