a method is described for the amperometric determination of the concentration in a biological sample of a constituent which is a substrate for an oxidase enzyme by catalyzing the substrate with the enzyme , thereby consuming o 2 and producing h 2 o 2 and measuring the o 2 consumption or h 2 o 2 production by means of an inert metal indicator electrode intimately coated with a thin film of a perfluorosulfonic acid polymer . a reaction chamber is described which provides means for contacting the sample with the enzyme and means for making amperometric measurements utilizing inert metal indicator electrodes coated with a thin film of a perfluorosulfonic acid polymer .