A study of photosynthesis of two grapevine cultivars, Vitis vinifera L. cv. Chardonnay and cv. Merlot in relation to the seasonal climate and internal CO2 (Ci) concentration at leaf temperatures from 15 to 45°C was undertaken. Average rates of photosynthesis at saturating CO2 concentrations and all leaf temperatures were higher in Merlot compared with Chardonnay leaves. This was attributable to higher rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and regeneration (Jmax) in Merlot leaves. These differences in photosynthesis were extended as the season progressed, partly because rates of RuBP carboxylation and regeneration of Chardonnay leaves declined markedly whereas rates for Merlot leaves remained high. Although there was no cultivar difference in the seasonal average temperature optima for assimilation (34°C) and the underlying metabolism (40°C for Vcmax and 35°C for Jmax), for temperatures above 35°C, the Merlot leaves had 50% higher rates. Across the season, activation energies of the temperature sensitivity of Vcmax and Jmax declined in response to the seasonal climate but were consistently lower in Merlot than Chardonnay. This suggested some apparent differences in the biochemistry occurred between the two cultivars that limited assimilation in Chardonnay leaves, especially at higher temperatures, but did not limit assimilation in Merlot leaves.