Some of the causes of the degradation occurring when cellulose ester plastic systems are exposed to excessive heat were investigated. It was found that the plasticizer is often the chief contributor toward break-down. Most of the common plasticizers were found to be more easily oxidized than the cellulose ester itself, and the oxidation process induced break-down of the chains. Antioxidants inhibited this type of break-down, whereas various metal soaps and titanium dioxide catalyzed it. In systems containing an oxidation-resistant plasticizer, a similar break-down could be produced by addition of a peroxide. A severe but nonoxidative type of break-down was found to occur when the plasticizer contained ferric salts of strong acids. This thermal break-down could be greatly reduced by the addition of a suitable chelating agent. In the process of break-down by ultraviolet light, oxygen also was observed to play an important part. The oxidizable plasticizer dibutyl sebacate was found to contribute to this type of break-down but to a far lesser extent than to break-down by heat. Ultraviolet-break-down inhibitors are discussed, and it is shown that antioxidants are useful only if combined with an additive that filters out the ultraviolet efficiently. In studies of weathering inhibitors, the thickness of the test piece is an important variable. Mechanisms by which ultraviolet break-down inhibitors may be effective are briefly speculated upon.