A method for estimating denitrification and nitrogen fixation simultaneously in coastal sediments was developed. An isotope-pairing technique was applied to dissolved gas measurements with a membrane inlet mass spectrometer (MIMS). The relative fluxes of three N₂ gas species (²⁸N₂, ²⁹N₂, and ³⁰N₂) were monitored during incubation experiments after the addition of ¹⁵NO₃. Formulas were developed to estimate the production (denitrification) and consumption (N₂ fixation) of N₂ gas from the fluxes of the different isotopic forms of N₂. Proportions of the three isotopic forms produced from ¹⁵NO₃ and ¹⁴NO₃ agreed with expectations in a sediment slurry incubation experiment designed to optimize conditions for denitrification. Nitrogen fixation rates from an algal mat measured with intact sediment cores ranged from 32 to 390 µg-atoms of N m² h¹. They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N₂ production rates due to denitrification could be separated from high N₂ consumption rates due to nitrogen fixation. Denitrification and nitrogen fixation rates were estimated in April 2000 on sediments from a Texas sea grass bed (Laguna Madre). Denitrification rates (average, 20 µg-atoms of N m² h¹) were lower than nitrogen fixation rates (average, 60 µg-atoms of N m² h¹). The developed method benefits from simple and accurate dissolved-gas measurement by the MIMS system. By adding the N₂ isotope capability, it was possible to do isotope-pairing experiments with the MIMS system.