Peatlands are important sources of atmospheric CH4, a potent greenhouse gas. Identifying the controls on CH4 flux from peatlands requires detailed investigation of fluxes at small scales (the microform scale) where flux chambers are best deployed for measurement. Using chambers presents challenges, not least that of how the CH4 concentration in chamber gas should be measured. Current field approaches usually involve either (i) online measurement with a flow-through laser absorption spectroscopy instrument or (ii) online or offline analysis of CH4 by gas chromatography–flame ionization detector (GC-FID) analysis. Online measurement restricts the number of chambers that can be monitored to c. 10 and their spatial spread to a distance of 10–20 m from the CH4 analyser and is not suited to large-scale studies. GC-FID may present logistical difficulties when conducting research at remote sites because of the need for frequent calibration and, hence, calibration gases. Generally, it is not suited to non-laboratory applications. An alternative is to analyse syringe samples of chamber gas near the field site using a modified laser spectroscopy instrument. We show how such a modification may be made. The theory of our modification is presented, as are the results obtained using it. It is shown that the instrument with the modification compares favourably with GC-FID and can be used near remote field sites in the absence of laboratory facilities. We present flux data from a peatland in Wales, where the flux-chamber CH4 concentrations were obtained using the modified instrument. Copyright © 2010 John Wiley & Sons, Ltd.