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.

Abstract: In a method of electrochemically sensing the presence of O.sub.2, N.sub.2 O and halothane in a fluid, a gold or platinum electrode and a silver electrode are exposed to the fluid through a permeable membrane.both electrodes are held at a first potential and the currents due respectively to reduction of oxygen at the gold or platinum electrode and reduction of oxygen and halothane at the silver electrode are sensed,the silver electrode is held at a second potential and the current due to reduction of oxygen and halothane and nitrous oxide is sensed,and from the three currents the presence and/or concentration of any of the three gases is determined.

Abstract: Carbon dioxide in the presence of oxygen is sensed by passage of the gas mixture through a layer of polymeric material and through a first, porous, working electrode in contact with a layer of aqueous electrolyte any oxygen being reduced at the electrode; any CO.sub.2 gas passes through the aqueous electrolyte and through a CO.sub.2 -permeable layer of polymeric material to a layer of non-aqueous solvent in contact with a second working electrode at which any CO.sub.2 gas is reduced, causing a current to flow.