The chemical composition and quantitative molar ratios among all components of biodegradable polyphosphoester copolymers Of DL-lactide and ethylphosphate were determined by a comprehensive set of NMR spectroscopic methods. The polyphosphoester copolymers studied were synthesized using condensation polymerization of oligomeric DL-lactide prepolymers and ethyl dichlorophosphate. Conclusive identification of the chemical shift patterns of all functional groups in the copolymers required additional NMR methods such as P-31-NMR and two-dimensional H-1-H-1 COSY NMR, in addition to the synthesis and comparative NMR analysis of model compounds possessing identical phosphoester linkages in the polyphosphoester copolymers. For the polymers synthesized using the bulk polycondensation process, H-1-H-1 COSY NMR analysis revealed the presence of a small amount of side products that were undetected by H-1-NMR alone. These side reactions most likely occurred between the pendant ethoxy group of the phosphoesters and the hydrogen chloride gas generated in the bulk polycondensation process. P-31-NMR spectra of the copolymers revealed a consistent triple-peak pattern characteristic of phosphoesters linked to a racemic mixture of D,L-lactides. These results offered new insight into the side reactions occurring in bulk polymerization of polyphosphoesters and provided a powerful tool of characterizing complex biodegradable polymers. (C) 2003 Wiley Periodicals, Inc.