There is described a method for measuring a concentration of a gas component in a measuring gas, wherein the light of a wavelength tunable light source is passed along a single optical path through a measuring volume containing the measuring gas and a reference cell containing a reference gas to a detector. The reference cell is selected to contain a selected isotope of the gas component to be measured in a known abundance ratio higher than the known natural-abundance isotope ratio of the gas component in the measuring volume; the light source is tuned to sweep the wavelength of the light over the absorption lines of the selected isotope and the remaining gas component; and the concentration of the gas component in the measuring volume is calculated from the ratio of the detector signals at the peaks of the absorption lines, based on Lambert's law and taking into account the known abundance isotope ratios.