A carbon monoxide sensor having a working electrode comprising a platinum oxide catalyst

Disclosed is an electrochemical sensor 1 for measuring carbon monoxide in a gas sample, the sensor comprising a working electrode 20 including a working electrode catalyst, and a diffusion restriction means 6 for restricting the diffusion of the gas sample to the working electrode; wherein the working electrode catalyst comprises oxidised platinum having a hydrogen cross-sensitivity of less than 10% and being present in an amount such that the activity capacity is at least 5. The electrode 20 may be formed from oxidised platinum. The oxidisation may be carried out via an oxidising agent, a heat treatment or anodising. The anodising may be applied with a pulsed potential. Also disclosed is a method of designing a gas sensor.