An IR sensor (1), especially a CO.sub.2 sensor, is described, having a filter arrangement (6), downstream of which there is arranged a detector arrangement (7), and an evaluating device (8) which is connected to the detector arrangement (7), the filter arrangement (6) having a first filter (9) and a second filter (10), which are configured as bandpass filters and each have a passband and of which the first filter (9) allows passage of a predetermined IR band and the second filter (10) does not, and the detector arrangement having two detectors (14, 15), each of which is associated with a filter (9, 10). The objective is to simplify the use of such an IR sensor. For that purpose, the passband of one filter (10) is arranged within the passband of the other filter (9) and the evaluating device (8) forms the difference of the signals (S1, S2) of the detectors (14, 15) and normalises it to the signal (S1) of a detector (14).