Abstract: A method of determining one gas in the presence of another, where the one gas is reactive with an electrochemical product of the other, comprisesapplying the gases to one side of a membrane permeable to the two gases, the other side of the membrane retaining a solvent for both gases and for the electrochemical product,applying, for the predetermined duration through a working electrode in contact with the solvent, a potential which electrolyses (e.g. reduces) the (dissolved) other gas to form the said electrochemical product,applying, through the working electrode, a potential for reconverting the product to the dissolved gas,measuring, at a predetermined instant or integrated over a predetermined time slot, the current flowing at the reconversion potential, and,from the amount by which the measured current (or integrated current, i.e. charge) falls short of the current when the said one gas is absent, determining the concentration of the said one gas.