Abstract: In a ratio-type double-beam infrared spectrophotometer, an output signal of the photodetector is frequency discriminated to derive therefrom a component having a fundamental frequency f corresponding to at least one of the sample and reference beam intensities and another component having a frequency 2f corresponding to at least the other beam intensity, and these frequency components are computed to determine the ratio of the sample beam intensity to the reference beam intensity. According to the invention, the component of fundamental frequency f is synchronously rectified into a rectified signal which is further differentiated into a differential value, the differential value is subtracted from the rectified signal to give a signal which is compensated for rapid changes of spectral absorption due to the presence of H.sub.2 O and CO.sub.2 in the beam paths during wavelength scanning and is used as the f-component signal.