A wavelength modulation spectroscopy method for measuring the concentration of a gas component is provided. A gas sample portion of the light is passed through a reference gas comprising the gas component in a constant concentration. Afterwards, the light is detected by a reference detector. Another portion of the light is passed through the gas sample and thereafter to a measuring detector. The light emitted by the light source is modulated with a frequency, while the wavelength is swept over a molecular absorption line of the gas component. Demodulation of the detector outputs is made at a higher harmonic. In order to compensate for variations of the modulation parameters of the light source in real time, a mathematical description of the demodulated reference detector output based on Fourier analysis of the modulated light and on a mathematical expression for the absorption line is provided.