A class of biodegradable polymers, poly (phosphoesters), was studied for its potential as a matrix for controlled drug delivery. The degradability of the phosphorus ester linkage and the versatility of phosphorus-containing polymers make poly (phosphoesters) attractive candidates for development as a degradable biomaterial. The release of various drugs in vitro from a series of bisphenol-A based poly (phosphoesters) was examined. Sustained release for up to several months was achieved for cortisone acetate and model drugp-nitroaniline. The mechanism of release was found to be a combination of diffusion, swelling and degradation. The structure of the sidechain influenced the release behavior in all instances.