We report on a colorimetric carbon monoxide (CO) sensor based on a rhodium complex. The sensor relies on changes in the light absorption properties of the chemochromic reagent when exposed to the gas. Its optical properties in solution and in a polymeric matrix were investigated by spectrophotometry. A strong color change from purple to yellow was obtained when exposed to CO. For its implementation in a sensor configuration, the complex embedded in a PVC-based matrix was spin-coated onto a glass waveguide. The sensor was tested in a gas cell and with standardized test fires. It showed a high sensitivity to low concentrations of CO, a fast reaction time and complete reversibility. The device may find applications in the field of fire detection and indoor air quality monitoring. (C) 2011 Published by Elsevier Ltd.