A physical gas sensor of thermal type capable of determining the concentrations of a gas mixture even in the presence of humidity by measuring the thermal diffusivity and conductivity includes a cell with thermal conductivity whose frequency response to the gases is known, in particular in respect of humidity, delivering a signal Vm representative of the concentration of the gas. This cell is excited by a pulsed signal so that a processing can be carried out at different analysis frequencies: at low frequency and at higher frequency. The output signals from the two processing chains are combined in the circuit (60) so as to provide the levels of concentration of the gas and the concentration of the water vapour. The signals obtained may also be re-combined together with components of the signal Vm of various passbands.