Factors that influence the open-circuit potential of a platinum/solid polymer electrolyte (SPE) electrode in air containing low concentrations of hydrogen were investigated. The electrodes were prepared from platinum in the form of a wire or a grid mechanically pressed, or a Pt layer chemically deposited, onto a Nafion(R) 117 membrane. The parameter underlying the behaviour of the PtSPE electrodes is the ratio of the rate of reduction of the platinum oxides to the hydrogen transport through a layer adjacent to the electrode surface, which can be controlled by, for example, the roughness factor of the platinum electrode. The lower is this ratio, the more sensitive is the electrode potential to the changes in the hydrogen concentration in the gas phase. Depending on the value of the roughness factor, the PtSPE electrode can be employed as either an indicator or a reference electrode. The design of a simple amperometric hydrogen sensor, which combines the Pt wire as an indicator electrode and the chemically deposited Pt as a reference electrode, is outlined.