Di-1-p-menthene, a film-forming antitranspirant, is reported to impede transpiration by acting as a physical barrier to water vapour loss from plant tissues. We tested this assumption on grapevine leaves across a range of temperature as well as on the bunch rachis and berries at several developmental stages. Methods and Results: Gas exchange of Shiraz leaves was monitored at 20, 25, 30 and 35°C under controlled conditions after treatment with 1 and 2% di-1-p-menthene. Transpiration, photosynthesis and stomatal conductance were reduced at each temperature, but instantaneous water use efficiency (CO2 fixation relative to water loss) was improved only at 25 and 30°C. There was no further reduction in water use or improved water use efficiency with the application of a 2% over 1% emulsion. The antitranspirant was most effective when applied to the underside of the leaves, forming a physical barrier over the stomatal cavity without reduction in stomatal aperture. Di-1-p-menthene also curtailed bunch water loss in Merlot by lowering transpiration of both the rachis and berries at the fruitset, peppercorn, pea size and veraison stages, and at 20 and 25°Brix. Increasing the concentration of the antitranspirant from 1% to 2 or 3% added some benefit; however, this was dependent on the developmental stage. Both leaf and berry temperature was elevated by the spray application. Di-1-p-menthene did not interfere with flowering or promote fruit abortion when applied prior to capfall. Conclusion: Di-1-p-menthene maintains berry turgidity through effects on both the canopy and the bunches themselves. Significance of the Study: The antitranspirant, di-1-p-menthene, may provide protection against desiccation under field conditions and this will have implications for yield and berry composition.