Many techniques for quantifying microbial biodegradation of C-labeled compounds use soil-water slurries and trap mineralization-derived CO(2) in solution wells suspended within the incubation flasks. These methods are not satisfactory for studies of arid-region soils that are highly calcareous and unsaturated because (i) slurries do not simulate unsaturated conditions and (ii) the amount of CO(2) released from calcareous soils exceeds the capacity of the suspended well. This report describes simple, inexpensive methodological modifications for quantifying microbial degradation of [C]benzene and 1,2-dichloro[U-C]ethane in calcareous soils under unsaturated conditions. Soils at 50% water holding capacity were incubated with labeled contaminants for periods up to 10 weeks, followed by acidification of the soil and trapping of the evolved CO(2) in a separate container of 2 N NaOH. The CO(2) was transferred from the incubation flask to the trap solution by a gas transfer shunt containing activated charcoal to remove any volatilized labeled organics. The amount of CO(2) in the trap solution was measured by scintillation counting (disintegrations per minute). The method was tested by using two regional unamended surface soils, a sandy aridisol and a clay-rich riparian soil. The results demonstrated that both [C]benzene and 1,2-dichloro[U-C]ethane were mineralized to release substantial amounts of CO(2) within 10 weeks. Levels of mineralization varied with contaminant type, soil type, and aeration status (anaerobic vs. aerobic); no significant degradation was observed in abiotic control samples. Methodological refinements of this technique resulted in total CO(2) recovery efficiency of approximately 90%.