Abstract: A method of determining the concentration of oxygen gas, [O2] and the concentration of carbon dioxide gas, [CO2], in a fluid, which method comprises: (a) applying the fluid to one side of a membrane permeable to the gases, the other side of the membrane retaining a solvent for the gases, (b) using a working micrelectrode in contact with the solvent to apply a first electric potential which is effective to reduce oxygen in the solvent and a second electric potential which is effective to reduce carbon dioxide in the solvent, (c) measuring a first steady-state limiting electric current, i1 corresponding to the reduction of O2 to O2.? and a second steady-state limiting electric current, i2 corresponding to both further reduction of O2 and reduction of CO2, and (d) de-convoluting i1 and i2 to determine [O2] and [CO2].