A fast-response, high precision, tunable diode laser spectrometer has been developed for field measurements of methane and nitrous oxide fluxes using the eddy correlation method. The instrument uses a novel multiple-pass absorption cell with astigmatic mirrors to provide a long absorption path length (36 m) in a small volume (0.31). The combination permits rapid response with sufficient sensitivity for eddy correlation flux measurements over a wide range of meteorological conditions. Signal processing uses a least-squares fitting algorithm to determine absolute trace gas concentrations directly from the absorption spectra using spectral line parameters at data rates up to 20 Hz. The precision for methane and nitrous oxide is 0.1% of ambient levels with a 1 s averaging time. Extensions of the method to measurements of surface-reactive gases such as nitric acid and ammonia are possible using rapid sampling techniques to minimize surface interactions.