Water permeability of isolated cuticular membranes from mature green and ripe Lycopersicon esculentum Mill, fruits was investigated. The water permeability of ripe tomato cuticles in the H+ form was strongly dependent on pH and increased when the pH of the external solution increased. The water permeability of the cuticles of mature green fruits was higher than that of the ripe fruit cuticles and had a low pH dependence. The isolated fruit cuticles were put in the Na+ form at three different pH levels and their water permeabilities were measured at the respective pH values in the external solution. The water permeability values were related to the nature and exchange capacity of hydroxyl functional groups of the flavonoids naringenin and chalconaringenin present in the ripe tomato cuticles, and to the phenolic acids of both types of cuticles. Whereas the water permeability of the mature green cuticles in the sodium form increased according to their small exchange capacity, the water permeability of ripe tomato cuticles reached a saturation value at high pH. The results indicate that the flavonoids play an important role in the control of water transport across the polymer matrix of these membranes. X-ray diffraction study of the different cuticles confirmed structural changes in the polymer when the isolated cuticles were put in the different homoionic forms. Analysis of the basal spacings confirms a structural model of tomato fruit cuticle with two major hydrophobic interplanar spaces around 1.0 and 0.45 nm wide.