An amperometric sensor is described which is suitable for the continuous monitoring of traces of hydrogen peroxide present in workplace atmospheres as gaseous species or as aerosols, without preliminary concentration steps being needed. It consists of a porous Pt sensing electrode, facing the sample, chemically deposited onto one side of a Nafion(R) 117 membrane which acts as a solid polymer electrolyte (SPE). The uncoated side of this membrane is contacted by an electrolyte solution (0.2 M acetate buffer at pH 4.8, +0.8 M NaClO4) contained in an internal compartment equipped with a Pt counter electrode and a Ag \ AgCl \ Cl-(sat.)(-) reference electrode. Hydrogen peroxide detection was achieved by applying a potential of 1.0 V to the sensing electrode, which was chosen on the basis of preliminary voltammetric investigations. The anodic current due to the oxidation of hydrogen peroxide increases linearly with its concentration within a wide range (from 0.1 to 40 ppm v/v) and a detection limit of 40 ppb v/v was estimated for a signal-to-noise ratio of 3. The estimated detection limit is about three orders of magnitude lower than I ppm. which is the recommended exposure limit for hydrogen peroxide in workplace atmospheres, evaluated as an 8-h time-weighed average concentration. Moreover, the long term stability of this amperometric sensor appears to be totally satisfactory in that reproducible responses were observed even after long periods of continuous use. The absence of significant interfering effects is also discussed.