Preliminary experiments revealed that the affecting impurities can be partially removed through vacuum calcination. While bio-polyP is labile upon heating under air at 120 • C as well as under ball milling conditions, affording orthophosphate and short-chain polyphosphates (see SI), calcination of bio-polyP at 500 • C under reduced pressure pyrolyzes organic impurities and removes the water content. The resulting dark material (Figure inset) cannot be effectively extracted with water, but a 31 P NMR spectrum of its aqueous extract still confirmed long-chain polyphosphate as the predominant form of P. Calcinated bio-polyP engendered an increased phosphite yield of 42%.