The phenolic composition of grapes and winemaking by-products (skins from grape marc, and lees) from Brazilian hybrid cultivars BRS Violeta (red) and BRS Lorena (white) was studied. Two vintages, five classes of phenolic compounds, and recovery yields using three dehydration techniques were considered: oven-drying at 50 °C (D50); spray-drying (SD); and freeze-drying (FD). Recovery yields were higher using FD, although less expensive SD was a good alternative for Violeta lees. D50 caused great recovery reduction in Violeta but yielded similar results for Lorena. Violeta winemaking by-products were excellent sources of anthocyanins (mainly non-acylated and p-coumaroylated diglucosides), flavonols (mainly myricetin-based) and hydroxycinnamic derivatives (mainly caffeic-based). Lorena winemaking by-products contained lesser amounts of phenolic compounds, around a tenth of the values found in Violeta grapes for flavonols (mainly quercetin-based), hydroxycinnamic derivatives (mainly caffeic-based) and condensed tannins. Lorena cultivar contained small amounts of trans-resveratrol and its 3-glucoside, which were missing in Violeta cultivar.