This document specifies a method to determine the specific aerobic biodegradation rate of solid, non-water soluble plastic materials under mesophilic conditions. The specific aerobic biodegradation rate (which, strictly speaking, is a specific mineralization rate, implying the assessment of the conversion of organic carbon into CO2 but neglecting biomass formation) is expressed as amount of carbon mineralized into CO2, per unit time, per unit area. The method described in this document does not provide information on the ultimate aerobic biodegradability of the tested samples. Biodegradability criteria for plastic materials under mesophilic conditions are provided for example by ISO/DIS 23517-1, and ISO 22403. The method described in this document can be used to determine the DT50 only when the plastic material is proven to be intrinsically biodegradable using suitable standard specifications such as ISO/DIS 23517-1 and ISO 22403. Furthermore, the biodegradation rate determined on plastic materials whose ultimate biodegradation has not been proven, cannot be considered as a specific characteristic of the whole material. This document only considers the evolution of CO2 as direct measurement of mineralization of the tested sample. The method described in this document can be applied also to solid materials used as a reference.