New biomedically erodible polymer composites were investigated. Polyphosphazenes containing the dipeptide side groups alanyl-glycine ethyl ester, valinyl-glycine ethyl ester, and phenylalanyl-glycine ethyl ester were blended with poly(lactide-co-glycolide) (PLGA) with lacto to glycolic acid ratios of 50 : 50 [PLGA (50 : 50)] and H : 15 [PLGA (85 : 15)] with solution-phase techniques. Each dipeptide ethyl ester side group contains two N-H protons that are capable of hydrogen bonding with the carbonyl functions of PLGA. Polyphosphazenes that contain only the dipeptide ethyl ester groups are insoluble in organic solvents and are thus unsuitable for solution-phase composite formation. To ensure solubility during and after synthesis, cosubstituted polymers with both dipeptide ethyl ester and glycine or alanine ethyl ester side groups were used. Solution casting or electrospinning was used to fabricate polymer blend matrices with different ratios of polyphosphazene to polyester, and their miscibilities were estimated with differential scanning calorimetry and scanning electron microscopy techniques. Polyphosphazenes with alanyl-glycine ethyl ester side groups plus the second cosubstituent were completely miscible with PLGA (50 : 50) and PLGA (85 : 15) when processed via solution-casting techniques. This suggests that the hydrogen-bonding protons in alanyl-glycine ethyl ester have access to the oxygen atoms of the carbonyl units in PLGA. However, when the same pair of polymers was electrospun from solution, the polymers proved to be immiscible. Solution-cast miscible polymer blends were obtained from PLGA (50 : 50) plus the polyphosphazene that was cosubstituted with valinyl-glycine ethyl ester and glycine ethyl ester side groups. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 431-437, 2010