Abstract: Biodegradable polymeric compositions are provided, wherein biodegradable polyphosphazenes are combined with at least one other polymer, either in the form of a blend, a semi-interpenetrating network (semi-IPN), or an interpenetrating network IPN. The side groups and composition of the polyphosphazenes are used to determine the properties of the compositions, for example, the rate and extent of degradation, and mechanical properties. These are useful in biomedical applications, including controlled drug delivery and tissue regeneration, and environmental applications. In the most preferred embodiment, as demonstrated by the examples, the polyphosphazenes contain hydrophobic side groups, such as p-methylphenoxy and other aromatic groups, and groups which impart hydrolytic instability, such as amino acid alkyl esters, and degrade by surface erosion. A preferred example is ethyl glycinato-substituted polyphosphazene (PPHOS) with p-methylphenoxy as co-substituent.