This document specifies a method for determining the ultimate aerobic biodegradability of plastic materials under controlled composting conditions by gravimetric measurement of the amount of carbon dioxide evolved. The method is designed to yield an optimum rate of biodegradation by adjusting the humidity, aeration and temperature of the composting vessel. The method applies to the following materials: natural and/or synthetic polymers and copolymers, and mixtures of these; plastic materials that contain additives such as plasticizers or colorants; water soluble polymers; materials that, under the test conditions, do not inhibit the activity of micro organisms present in the inoculum. If the test material inhibits micro organisms in the inoculum, another type of mature compost or pre exposure compost can be used.