We investigated how foliar application of kaolin particle film influenced diurnal leaf gas exchange, leaf water potential, yield, and berry maturity of a red (‘Merlot’) and white (‘Viognier’) wine grape (Vitis vinifera L.) cultivar under differing levels of water stress over two growing seasons (2005 and 2006) in the warm, semiarid climate of southwestern Idaho. Net diurnal stomatal conductance (gS) was increased by particle film and the effect varied according to vine water status. Particle film delayed the onset of diurnal decline in gS under mild water stress (leaf water potential –1.2 MPa) but had no influence on leaf gas exchange when vines were under greater water stress (leaf water potential –1.4 MPa). Correlation between soluble solids concentration and titratable acidity (‘Viognier’) and between berry fresh weight and yield (‘Merlot’) was higher with than without particle film, suggesting that particle film may attenuate the influence of other factors affecting expression of these traits. Particle film was associated with an increase in berry weight in ‘Merlot’ and with an increase in berry soluble solids concentration in ‘Viognier’, suggesting that the film may increase vine-carrying capacity. Midday leaf water potential throughout the growing season was not influenced by particle film. Fruit surface browning was observed on deficit-irrigated, particle film-treated vines on exposed clusters on the west side of the canopy, indicating that the film did not eliminate development of heat stress symptoms on fruit under the most extreme environmental conditions evaluated in this study. (We recommend that you consult the full text of this article.)