The hydrolysis of tripolyphosphate to orthophosphate is facilitated by suspensions ofamorphous manganese dioxide. Pyrophosphate is observed as an intermediate. The rate enhancementdecreases with increasing pH. The weak sorption of orthophosphate on the oxide also decreases withincreasing pH, indicating that the orthophosphate product would be bioavailable. The presence of calciumions in natural waters increases both the rate of hydrolysis of tripolyphosphate and the extent of sorptionof orthophosphate. Under some circumstances this abiotic mechanism could play a significant role in thehydrolysis of tripolyphosphate in the aquatic environment.