Suspensions of two strains of Pseudomonas aeruginosa (ON12 and ON12-1) were used to reduce NO3- and NO2-, respectively, to N2O. The evolved N2O was quantified by gas chromatography with electron capture detection, and the 15N abundance was determined by mass spectrometry with a special inlet system and triple-collector detection. Sample gas containing unknown N2O pools as small as 0.5 ng of N was analyzed by use of a spike technique, in which a reference gas of N2O of natural 15N abundance was added to obtain enough total N for the mass spectrometer. In NO3- or NO2- pools, the 15N abundance could be determined in samples as small as approximately 3.5 ng of N. No cross-contamination took place between the NO3- and NO2- pools. The excellent separation of NO3- and NO2- pools, small sample size required, and low contamination risk during N2O analysis offer great advantages in isotope studies of inorganic N transformations by, e.g., nitrifying or denitrifying bacteria in the environment.