High temperatures during the growing season characterise many grape growing regions in Australia and elsewhere in the world, and impact on many processes including growth and berry development. To quantify the impact of heat on the Vitis vinifera L. cv. Semillon, potted vines were grown in controlled environments and exposed to a temperature regime of 40/25°C at flowering, fruit set, veraison and mid-ripening stages. Vegetative and reproductive development was measured throughout and leaf photosynthesis and stomatal conductance tracked during heat exposures. Accumulation of soluble solids was determined during ripening. Leaf growth and stem extension were unaffected by heat whereas flowers completely abscised. Berries treated at fruit set developed normally and those treated at veraison and mid-ripening stopped expanding and sugar content stopped increasing. Photosynthesis was also affected on each occasion, with rates declining by 35% and taking 12 days to recover. Up to 10 mg carbon g (berry dry weight)'1 day'1 was required for ripening after veraison. For vines heat treated at veraison and mid-ripening, net carbon acquisition rates fell to below 4 mg carbon g (leaf dry weight)'1 day'1, which is inadequate to supply berry carbon requirements. This suggests that the impacts of heat on the ripening process can be traced back to the supply of carbon.