Abstract: A probe for detecting variations in the composition of a mixed-gas flow, designed primarily for monitoring variations in the composition of exhaust gases of an internal combustion engine for anti-pollution purposes and based on the pile effect in a solid electrolyte, one side of which is exposed to the gas being tested and the other to a reference gas, the solid electrolyte having the form of a tube open at its ends and traversed by the reference gas passing through its interior, while its outer surface is in contact with the gas flow under test. The electrolyte tube is inserted transversely into the flow passage, its reference gas being released into the passage through the orifice in the free end of the tube.