A potentiometric sensor has been fabricated for determining hydrogen activity at the surface of mild steel at room temperature. The electrochemical cell comprises : Metal salt reference electrode solid electrolyte steel. The potential is measured by electrical contact of the sensor’s reference electrode terminal and steel to a high impedance voltmeter. Interfacial contact of an exposed section Of the sensor’s deformable electrolyte to steel was assured by a magnetic collar around the sensor head, to which silicone grease had been previously applied. The sensor response time is several tens of seconds. We have observed stable sensor potentials of 300 to 320 mV versus SHE with untreated mild steel coupons ground with dry P280 SiC paper, though markedly more reducing potentials were obtained initially when preparative conditions such as grinding in deionised water favoured hydrogen occlusion. Dry ground steel coupons which had been previously immersed in 17 vol% sulphuric acid containing 0.1 M zinc sulphide for 20 h typically yielded cell potentials -250 mV. Subsequent hydrogen depletion at the steel surface was suggested by the cell potential moving positively to +300 mV over the course of several days. When the sensor was attached to charged and untreated steel alternately, the minimum cell potential of +150 mV with the untreated steel indicated a hydrogen activity carried over from the charged steel in the sensor electrolyte or trapped grease not exceeding 10(-5) atmospheres.