This paper describes the validation and application of a simple flask-based C-14-respirometer system designed to assess mineralisation of C-14-labelled substrates under defined conditions. Validation of this respirometer system indicated stoichiometric CO2 trapping up to a maximum of 400 mu mol of CO2 tin a single trap). Polycyclic aromatic hydrocarbon (PAH)-degrading bacteria were used to measure growth-linked biodegradation of [C-14]naphthalene to (CO2)-C-14. A C-14 activity balance of 101.7 +/- 8.9% (n = 6), after 74 h incubation time and 10 respirometer-opening events, indicated the suitability of the system for monitoring substrate mineralisation. This respirometric apparatus was then successfully applied to assess: (i) the PAH catabolism of microbes in a field contaminated soil, where naphthalene and phenanthrene were rapidly mineralised and (ii) soil-associated organic contaminant bioavailability, where increased soil-phenanthrene contact time resulted in a reduction in phenanthrene mineralisation in the soil. The described respirometer system differs from existing respirometer systems in that the CO2 trap can be removed and replaced quickly and easily. The system is efficient, reproducible, adaptable to many situations, easy to construct and simple to use, it therefore affords advantages over existing systems. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.}