We identified two glycosyltransferases that contribute to the structural diversification of flavonol glycosides in grapevine (Vitis vinifera): glycosyltransferase 5 (Vv GT5) and Vv GT6. Biochemical analyses showed that Vv GT5 is a UDP-glucuronic acid:flavonol-3-O-glucuronosyltransferase (GAT), and Vv GT6 is a bifunctional UDP-glucose/UDP-galactose:flavonol-3-O-glucosyltransferase/galactosyltransferase. The Vv GT5 and Vv GT6 genes have very high sequence similarity (91%) and are located in tandem on chromosome 11, suggesting that one of these genes arose from the other by gene duplication. Both of these enzymes were expressed in accordance with flavonol synthase gene expression and flavonoid distribution patterns in this plant, corroborating their significance in flavonol glycoside biosynthesis. The determinant of the specificity of Vv GT5 for UDP-glucuronic acid was found to be Arg-140, which corresponded to none of the determinants previously identified for other plant GATs in primary structures, providing another example of convergent evolution of plant GAT. We also analyzed the determinants of the sugar donor specificity of Vv GT6. Gln-373 and Pro-19 were found to play important roles in the bifunctional specificity of the enzyme. The results presented here suggest that the sugar donor specificities of these Vv GTs could be determined by a limited number of amino acid substitutions in the primary structures of protein duplicates, illustrating the plasticity of plant glycosyltransferases in acquiring new sugar donor specificities.