Field measurements of photosynthesis of Vitis vinifera cv. Semillon leaves in relation to a hot climate, and responses to photon flux densities (PFD) and internal CO2 concentrations (ci) at leaf temperatures from 20 to 40oC were undertaken. Average rates of photosynthesis measured in situ decreased with increasing temperature and were 60% inhibited at 45oC compared with 25oC. This reduction in photosynthesis was attributed to 15-30% stomatal closure. Light response curves at different temperatures revealed light-saturated photosynthesis optimal at 30oC but also PFDs saturating photosynthesis increased from 550 to 1200 Âµmol (photons) m-2 s-1 as temperatures increased. Photosynthesis under saturating CO2 concentrations was optimal at 36oC while maximum rates of RuBP carboxylation (Vcmax) and potential maximum electron transport rates (Jmax) were also optimal at 39 and 36oC, respectively. Furthermore, the high temperature-induced reduction in photosynthesis at ambient CO2 was largely eliminated. The chloroplast CO2 concentration at the transition from RuBP regeneration to RuBP carboxylation-limited assimilation increased steeply with an increase in leaf temperature. Semillon assimilation in situ was limited by RuBP regeneration below 30oC and above limited by RuBP carboxylation, suggesting high temperatures are detrimental to carbon fixation in this species.