This investigation elucidates some aspects of auxiliary chemicals on the biodegradation of two commercial polyurethanes (Pellethane and Corethane). The materials were incubated for 28 days with cholesterol esterase and/or with phosphatidylcholine. Extraction studies were carried out on the two materials, using different solvents, chosen on the basis of solvent polarity. FT-IR spectra for the extracted materials indicated the presence of poly(methylene)n oxide moities, silicone oil, bis-ethylene-stearamide, aromatic moities, and alkyd-urea compounds in Pellethane. Corethane materials were shown to contain some fatty acids, hydrocarbon waxes, ester-based species, and chlorinated compounds. Analysis of incubation solutions by high performance liquid chromatography failed to isolate methylene dianiline (MDA) or any of its derivatives from the various polymer incubation solutions. However, a methanol extract of Corethane samples that were incubated for 28 days in cholesterol esterase did show the presence of MDA. The absence of MDA in the Pellethane methanol extracted samples may reflect the differences in surface additives found for this material versus the Corethane. FT-IR/ATR analysis of polymer surfaces exposed to cholesterol esterase/phospholipids mixture showed that there was an increase in the uptake of phospholipids over samples that were incubated in phospholipid dispersion alone. The results of this study show that some of the auxiliary chemicals found in commercial polyurethanes may hinder the specific release of hydrolytic degradation products and delay polymer degradation. However, it should be recognized that the surface layer containing these compounds is susceptible to change following the interaction between the polyurethane-based devices and elements of the host environment (i.e. lipids, enzymes, etc.). Hence, recognition and identification of these changes will ultimately be important in assessing a commercial polymer's blood compatibility characteristics.