Novel biodegradable materials with tunable hydrolytic degradation rate are prepared by grafting of phosphonoethylated polyglycidols with polyesters. First, the hydrolytically degradable polyester grafts are attached to polyglycidols partially grafted with phosphonoethylated diethyl esters through chemical-catalyzed grafting using tin(II) octanoate, then the diethyl ester groups are chemoselectively converted to the corresponding monoester (mixed phosphonate/phosphonic acid) using alkali metal halides. The products are characterized by means of (1)H, (13)C, and (31)P NMR spectroscopy, as well as size-exclusion chromatography and differential scanning calorimetry. The in vitro degradation of the copolymers is studied in phosphate buffered solution at 55 °C. The copolymers are of the same architecture, molecular weight, and crystallinity, only differing in the pendant phosphonate and mixed phosphonate/phosphonic acid groups, respectively. On the basis of mass loss, decrease of the molecular weight, and morphological analysis of the copolymers, the strong impact of mixed phosphonate/phosphonic acid groups on the hydrolytic degradation rate is demonstrated.