Kinetics of hydrolysis were studied for two polyesters of phosphoric acid, namely poly(methyl ethylene phosphate) and poly(trimethylene phosphate). The rate of hydrolysis was determined by direct titrimetric and NMR methods. The latter allowed establishment for poly(methyl ethylene phosphate) the rates of hydrolysis of the main chain (k(m)) and the side methyl group (k(s)). The rate constants determined for polymers have been found to be close to those measured for the corresponding low molecular weight models in the pH range from 1 to 12. In acidic conditions the methyl group hydrolyzes faster, whereas at basic conditions, both the methyl group and the main chain depart with approximately similar rates. This difference stems from different mechanisms of hydrolysis : in the acidic conditions the carbon atoms are attacked, whereas in the basic conditions the phosphorus atom is attacked. On the basis of kinetics of hydrolysis, the time required to break 1% of bonds in the macromolecule changes from similar to 150 h at pH similar to 2.0 to similar to 40 h at pH similar to 7.8 (both at 45 degrees C).