The biodegradation kinetics in soil of a biodegradable multi-constituent plastic material have been determined with a standard test method based on the measurement of the evolved CO₂. Three different kinetics were identified, possibly corresponding to (i) the biodegradation of low molecular weight constituents, (ii) the self-degradation of biomass formed in the first phase, (iii) the biodegradation of the bulk polyesters. The relationship between surface area and mineralization rate was determined using regression analysis. The regression model suggests that if it were technically possible to test the plastic material in the form of nanoplastics (spheres of 100 nm diameter) it would take 15–20 days to reach full biodegradation, a time frame compatible with the OECD requirements for readily biodegradable chemicals. The specific mineralization rate of test material was estimated to be 0.003439 mg organic carbon/day/cm². We put forward the testing approach applied in this work as a means to characterize biodegradable plastics and obtain constants relevant for eco-design and for environmental fate studies.