Dissolved gas concentration in environmental water samples is usually determined by headspace equilibration (HE) techniques followed by laboratory analyses. This method generally causes a time delay between field sampling and measurements that (i) impedes feedback of results that would otherwise inform field sampling strategies, (ii) may affect the precision of the results due to biological activity or leakage, and (iii) may require logistically complex procedures for transport and storage of samples. The development of an on-site measurement technique combined with headspace equilibration methods would be beneficial, especially for multiple field measurements in remote areas. In this article, we present a method, based on headspace equilibration but using infrared tunable diode laser absorption spectroscopy (HE-TDLAS) that allows immediate determination of dissolved gas concentrations in environmental water. HE-TDLAS method was first tested in the laboratory and compared with a gas chromatography technique. No significant difference was observed between both methods. HE-TDLAS method was then used during field experiments for the measurement of dissolved methane concentration in 110 water samples from several Alaskan lakes. Dissolved methane concentrations ranging from 0.014 to 20 mg L-1 were successfully measured with standard errors of 28% to 2%, respectively. A quantification limit of 13.6 mu g L-1 and an accuracy of 3.6% were established from the statistical analysis. Results suggest that the HE-TDLAS method is applicable to many aquatic ecosystems, including wastewater treatment facilities, landfills, rice paddies, and freshwater ecosystems, in which dissolved methane typically occurs in the mu g to mg L-1 range.