AbstractWater penetrated through isolated leaf cuticles of dwarf orange (Citrus mitisBlanco, ‚Calamondin’︁) as undissociated molecules because both [18O] water (1H218O) and HTO (1H3H16O) permeated at the same rate. HTO penetrated to 3 to 21% of the theoretical equilibrium value (TEV) in an unstirred system within 10 days for astomatous cuticles and 50 to 60% of TEV for stomatous cuticles. The permeability coefficient (k) of HTO through astomatous cuticles at 25°C was 6.8 × 10−7cm s−1. Two highly water‐soluble14C‐labelled compounds, trichloroacetic acid (TCA) and 1,2,4‐triazol‐3‐ylamine [aminotriazole (BSI) or amitrole (ISO)], and two nearly water‐insoluble14C‐labelled compounds, 1‐naphthyl methylcarbamate (carbaryl) and 2,6‐dichloro‐4‐nitroaniline (dicloran), were compared to HTO as a reference standard in permeation studies. All four organic molecules permeated without decomposing. The relativekvalues for TCA, aminotriazole, carbaryl, HTO and dicloran were 0.32, 0.47, 0.71, 1.0, and 1.5 respectively. Although this suggested that the permeation of organic molecules may be inversely related to water solubility, this could not be established with certainty due to large variations in the data. Thekvalues were obtained for 12 other organic compounds through a variety of biological and model membranes or were calculated from the literature. Any relationships betweenkand various molecular characteristics were unclear because a wide variety of cuticle sources and experimental design was used by different investigators working in this area. The calculation ofkis considered essential in all permeability studies so that comparisons