A small, low-cost sensor for measuring orbital atomic oxygen fluxes and their corrosive effect on spacecraft materials has been developed. Its operation is based on the reaction between hyperthermal atomic oxygen and a thin silver film. The electrical conductivity of the silver oxides formed is negligible allowing the reaction rate to be monitored through resistance measurements of the film. Atomic oxygen effects on spacecraft coatings can be investigated by placing a sample above the silver film. The construction of the sensor emphasizes simple techniques and components to improve reliability and reduce costs, while its small size and mass will allow it to be used on a wide range of spacecraft, increasing flight opportunities. An instrument comprising 12 of the sensors has been tested in orbit on the Space Technology Research Vehicle (STRV-1a) microsatellite mission. Results from this experiment agree well with those from ground-based simulation facilities. The successful outcome of this demonstration flight shows that the sensor can operate successfully and usefully in space. Several improvements to the technique are suggested for follow-on flights. ©1997 American Institute of Physics.