Ring-opening polymerization of cyclic phosphates offers a fast access to well-defined, water soluble and (bio)degradable polyphosphoesters (PPEs). In particular, poly(alkyl ethylene phosphate)s have been used as building blocks for nanocarriers or hydrogels. The molecular mechanism of their degradation is, however, not well understood. Herein, we study the hydrolytic degradation of two most frequently used PPEs, poly(methyl ethylene phosphate) (PMEP) and poly(ethyl ethylene phosphate) (PEEP). The degradation process is analyzed by NMR spectroscopy, which identifies and quantifies intermediates and degradation product(s). We prove that the major degradation pathway is backbiting, leading to one dominating hydrolysis product, ethyl or methyl ethylene phosphate (a diphosphate). Accelerated hydrolysis, performed in basic and acidic conditions, shows the high stability of PEEs under acidic conditions, while they readily degrade under basic conditions. The backbiting mechanism is further supported by the reduction of the degradation kinetics if the terminal OH-group is blocked by a urethane. Our findings help to develop degradable nanodevices with adjustable hydrolysis kinetics.