Four commercially produced starch based plastics manufactured by the addition of starch to polyethylene matrix were tested in the laboratory to measure their biodegradability under both aerobic and anaerobic conditions. Pure starch was also tested under identical conditions to compare rates and extent of degradation of starch in the plastics to that of pure starch. Plastics surface properties and weight average molecular weights were observed before and after biological tests using Scanning Electron Microscopy (SEM), and High-Temperature Gel-Permeation-liquid Chromatography (HT-GPC) (Lee et al., 1991) respectively. Degradation rates and total percentage of carbon conversion were higher under aerobic tests as compared to tests under anaerobic conditions. Overall carbon conversion in starch based plastics was less than that of pure starch. Surface analysis of the plastics indicated removal of some starch granules from the polyethylene matrix. Significant changes did not occur in the average molecular weights of the plastics from HT-GPC tests which indicated no degradation of the polyethylene matrix.