Oxygen is both product and substrate of photosynthesis and metabolism in plants, by oxygen evolution through water splitting and uptake by photorespiration and respiration. It is important to investigate these processes simultaneously in leaves, especially in response to environmental variables, such as light and temperature. To distinguish between processes that evolve or take up O2 in leaves in the light, in vivo gas exchange of stable isotopes of oxygen and membrane inlet mass spectrometry is used. A closed-cuvette system for gas exchange of leaf disks is described, using the stable isotopes 16O2 and 18O2, with a semipermeable membrane gas inlet and isotope mass separation and detection by mass spectrometry. Measurement of evolution and uptake, as well as CO2 uptake, at a range of light levels allows composition of a light–response curve, here described for French bean and maize leaf disks.