The present invention relates to a gas sensor (100) comprising a measurement chamber (102) for measuring presence of a first gas, the first gas being nitrous oxide (N2O) and an optical source (103) for emitting radiation in the measurement chamber (102). The sensor also comprises a radiation detector (104) sensitive to radiation emitted by the optical source (103), the radiation being detected by the radiation detector (104) following passage through the measurement chamber (102). A diffusion layer (131) is configured to allow diffusion of ambient gas into and out from the measurement chamber (102), and the diffusion layer (131) comprises means (133) for preventing diffusion of carbon dioxide (CO2) in the ambient gas into the measurement chamber (102). The invention also relates a nitrous oxide detector comprising such a gas sensor, and a method for determining presence of nitrous oxide in the ambience.