During 2005 and 2006 four irrigation treatments were assayed in a vineyard growing 'Merlot' grapes lo-cated in a warm climate region. The treatments were kept for two years and the wine was produced the sec-ond year so that the response of the wines to water con-straint was consistent. The phenolic parameters of the wines were analysed. The phenolic composition of the seeds was studied for both years and more significant differences were observed in the second. The results showed that when the deficiency increased, so did the concentration of total polyphenols, flavan-3-ols and tannins in the seeds. Pre-dawn leaf water constraint integrals between 20 and 24 Mpa as opposed to levels of 9 to 14 caused statistically significant increases in total polyphenols, flavan-3-ols and the colour index of the wine. These differences were manifested in sensory terms by significant intensification of the colour and body of the wines. K e y w o r d s : water constraint, phenolic composition, colour, Vitis vinifera L. cv. Merlot, wine.