Novel water-soluble biodegradable polyphosphazenes are described. Polymers are constructed on the basis of a biodegradable polyphosphazene backbone to mimic the structure of poly(vinylpyrrolidone), PVP. Poly{di[2-(2-oxo-1-pyrrolidinyl)ethoxy]phosphazene}, PYRP, and its copolymers containing biologically active carboxylatophenoxy side groups have been synthesized. The ability of these polymers to degrade in aqueous environment at 37 degrees C is demonstrated by light scattering-GPC and NMR studies. The degradation is manifested by the gradual decrease in the molecular weight and release of the side group -1-(2-hydroxyethyl)-2-pyrrolidone. It has been shown that the incorporation of N-ethylpyrrolidone containing side groups in the polymer structure can modulate degradation profiles of phosphazene copolymers. The kinetics of hydrolytic degradation and the effect of pH on the degradation rate have been investigated.