The invention relates to a NDIR two beam gas analyser in which infrared radiation is guided by modulation in an alternating manner through a measuring chamber and a reference chamber and is subsequently detected, a measurement signal being produced due to the analysis which determines the concentration of a measurement gas component present in the measurement chamber. The detection and compensation of error effects, in particular modifications on the infrared radiation source or detector arrangement, is simplified as a phase imbalance is produced in the switching of the radiation between the chambers, and the measurement signal is detected in a phase-sensitive manner for modulating the radiation, a measurement signal vector (SF) comprising amplitude information and phase information (ΦF) is obtained such that during calibration of the gas analyser for different known concentrations (K1, K2, K3, K4, K5) of the measurement gas components, measurement signal vectors (S1, S2, S3, S4, S5) having different amplitudes and phases are determined, vectors define a characteristic line (43), and when an unknown concentration of the gas component is measured, the unknown concentration of the measurement gas component is determined from the intersection point (45) of an obtained measurement signal vector (SF) or the extension thereof with the characteristic line (43).