A laboratory method is specified for determining the aerobic biodegradability of plastic materials. In aqueous medium within a closed respirometer, the material under test is exposed to an inoculum derived from activated sludge, compost or soil, and the oxygen demand is measured. If unadapted activated sludge is used, the test simulates the natural biodegradation process; use of adapted or mixed inoculum permits the potential extent of biodegradation to be estimated. The test is applicable to: natural and/or synthetic polymers, copolymers or their mixtures; plastics containing plasticizers, colorants or other compounds; water-soluble polymers; materials which do not inhibit the microorganisms present in the inoculum.