The head-space test based on CO2 measurement was improved as a test for determining of the biodegradability of solid polymers, such as biodegradable plastics. The test can be performed either as closed or aerated cultures depending on the size of the sample and its oxygen demand. The distribution of CO2 between the gas and liquid phases was also determined during the cultivations. The amount of microbial carbon was determined indirectly by determining the carbon/protein ratio and then using the intracellular protein as an indirect measure of the biomass. The amount of biomass was found to be so low that its determination did not significantly effect the final biodegradation result.