The biodegradation of a coextruded starch/ poly(lactic acid) polymeric material was studied in a liquid, in an inert solid, and in composting media. Mineralization of the material's carbon content was followed using the appropriate ASTM standard experimental methods. The final percentage of biodegradation depended on the nature of the medium used. The percentage of mineralization (Cg) was better with the liquid medium (65%) than with either the compost medium (64%) or the inert medium (59%). To understand the workings of the biodegradability of the polymer material, a measurement of the carbon balance seemed essential A repartitioning of the material's carbon between the various degradation products to biomass carbon (Cb), to carbon dioxide carbon (Cg), to dissolved organic carbon (Cs), and to residual insoluble material carbon (Cnd) produced was quantified throughout the experimental runs. (C) 2003 Wiley Periodicals, Inc.