Stomata are small pores on leaf surfaces that regulate gas exchange. When stomata are closed evapotranspiration is reduced. This means that the temperature of leaves with closed stomata is warmer than leaves with open stomata. It is possible to visualise these temperature differences with an infrared thermal imaging camera. We are currently using this approach to isolate Arabidopsis stomatal CO2 response mutants. To do this we built a Teflon covered chamber that allows plants to be imaged from the outside. Once plants are inside the chamber we recorded thermal images during a period at ambient [CO2] (360 ppm) followed by a period at elevated [CO2] (up to 1400 ppm). Results showed that leaf temperature of 25 to 35 days old wild type plants increased 1–2 -C following exposure to elevated CO2. We screened around 18,000 EMS plants and plants that did not show the characteristic response to high CO2 were selected and seed collected. We are currently characterizing and analysing these putative mutants.