A gas sensor for determining a concentration of at least one gas in a gas mixture includes: at least one intensity modulatable light source; a measuring section, into which the gas mixture to be investigated can be allowed to flow; and an essentially gas-sealed detection cell, wherein the gas sensor is embodied such that light emitted from the light source is radiated into a measuring section, wherein the intensity of the emitted light is modulated with a modulation frequency, which differs from the resonant frequency of a mode of the acoustic resonance of the detection cell by less than 0.5 times, especially less than 0.25 times, the half-width of the mode. Further, a method for determining the concentration of the at least one gas in the gas mixture uses the gas sensor.