The evaporation of water through a plastic membrane coated with plant was (30-70 μg cm(2)) from grape berries or fractions thereof was determined. The hydrocarbon, alcohol and aldehyde fractions caused the highest reduction of evaporation. Their effect was identical to the complete wax or to mineral paraffin wax. The main constituent of the grape cuticle wax, the triterpene oleanolic acid, had no effect on evaporation in the artificial system. Free docosanoic acid did not suppress evaporation whereas the mixture of free fatty acids (the main constituents are the C24 and C26 acids) from grape wax reduced evaporation slightly. The results from this artificial system suggest that the alcohol, hydrocarbon and aldehyde fractions are the active components of the grape cuticle which prevent water loss.