Determination of C-14-marked methane by gas chromatography and liquid scintillation counting is shown to be useful in studies of isotope effects. Data on the specific activity is used to separate the contributions of (CH4)-C-14, and (CH4)-C-12 to the gas-chromatographic peak area. As an application the difference in diffusion for (CH4)-C-12 and (CH4)-C-14 is observed. The total efficiency is determined without resort to a radioactive standard by utilising information on the specific activity. (C) 1997 Elsevier Science Ltd.