Changes in phenolic composition were determined in cv. Syrah-vines grape skins during ripening for two contrasting yields, resulting from cluster thinning at veraison. Treatments consisted on 16 and eight clusters per plant leading to approximately 8 ton/ha (T1) and 4 ton/ha (T2). In the grape skins of the samples analysed, 11 different low molecular weight phenolic compounds were identified, as well as 15 anthocyanins. Cluster thinning had a minimal effect on ripening time and weight of grape skins, however, clusters from low yield plants resulted in a lower total acidity and slightly higher pH. As for the phenylpropanoid pathway, the flavan-3-ol (+)-catechin, and the flavanols isorhamnetin-3-O-galactoside and isorhamnetin-3- O-glucoside, resulted in a higher concentration in berry skins from low yield plants. It is concluded that cluster thinning may result, from the oenological point of view, in an increased grape quality especially in compounds related to wine colour.