Abstract: An organic fluid detection instrument having great sensitivity to low concentrations or organic fluids in a fluid stream comprise a conduit adapted for the one-way flow therethrough a mixture of an oxygen-containing carrier fluid and organic fluids; a first electrical resistance element disposed in the conduit and carrying a material catalyst for promoting exothermic oxidation reactions of the organic fluid of the mixture; a second resistance element also disposed in the conduit for temperature sensing, consisting of a material with an electrical resistivity that varies with its induced temperature; each element is connected to a pair of electrical leads for operatively producing electrical signals representative of the amplitude of the output signal to be measured; a measuring circuit means is joined to both pairs of leads for processing both a synchronous signal and the second output signal in a manner whereby variations in the differential resistance of the second sensing element serve as a measure of the