The effects of 0.15 ppm sulphur dioxide and (or) 0.15 ppm ozone on membrane permeability, measured by electrical conductivity of solute leakage, were determined for soybean, white bean, cucumber, and radish. Solute leakage increased significantly prior to visible injury for soybean and white bean exposed to 0.15 ppm ozone alone. This results in water-soaked lesions on the upper leaf surface prior to visible coloured necrotic lesions and is representative of a "leaky" plasmalemma. The sulphur dioxide – ozone mixture did not increase solute leakage above that of the control for white bean and soybean. Therefore, the presence of sulphur dioxide, in combination with ozone, attenuates or reduces the effects of ozone on increasing membrane permeability of white bean and soybean. Ozone alone did not affect membrane permeability in cucumber and radish. The sulphur dioxide – ozone mixture, which produced more injury than ozone alone, did increase membrane permeability temporarily for radish and cucumber but this occurred only after visible injury symptoms appeared. Overall, the effects of the pollutants singly or in combination on membrane permeability appear to offer an explanation of differences in species responses.