We describe an atmospheric oxidation sensor developed and delivered as part of the European Space Agency (ESA) Beagle 2 (B2) Mars Lander Environmental Sensor Suite (ESS). The sensor monitors atmospheric oxidation rates by measuring resistance changes in a thin-silver (Ag) film deposited in situ onto a sapphire substrate while on the surface of Mars. Potential terrestrial applications of this sensing approach include long-term monitoring of oxidative contaminants in low-oxygen systems including process gases and environmental chambers. The sensor response to ppb levels of hydrogen peroxide (H 2O 2) vapor in carbon dioxide (CO 2) is demonstrated.