Abstract: It is presented a method for detecting a gas (G). Acoustic waves (Wt) are generated and transmitted via a wave generating and sensing means (2) towards a reflecting wall (3) and thereafter reflected acoustic waves (Wt) are detected by the wave generating and sensing means (2) wherein a presence of the gas (G) is detected by determining a change in an output signal of the wave generating and sensing means (2). A gas detector (1) is also presented.