A field portable, stead-state gas-exchange system which measures both CO2 and water vapour exchange of single intact leaves during fumigations with SO2 is described. Within the leaf cuvette temperature, light, humitity and both CO2 and SO2 concentrations are controlled to preset levels. Gas flow and concentrations are controlled by mass flow controllers. Photosynthetic uptake of CO2 can be determined either by differential depletion or null balance measurement. Water vapour exhange is measure differentially and transpiriation and conductance to water vapour determined. Sulphur dioxide is measured directly within the cuvette exhaust gas line by UV-pulse fluorescence. The performance of this system under field conditions is described and the physiological measurements compare dwith those obtained with other systems.