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Three peach (‘Honora’, ‘Dr. Davis’ and ‘Fairtime’) and five nectarine (‘Maria Anna’, ‘Diamond Ray’, ‘Fairline’, ‘Nectaross’, ‘Sweet Red’) cultivars were analyzed at harvest and after a postharvest ripening period. Physicochemical characteristics [peel ground color (L*,C*, h°), soluble solids content (SSC), flesh firmness and titratable acidity (TA)], the concentration of some bioactive compounds [total phenol content (TPC) and total carotenoids (TC)] and the total antioxidant activity (TAA) were evaluated at harvest and after a shelf-life period of five days at 20 °C. Phenolic compounds and antioxidant activity were assayed on two different extracts of each sample: ethanol/HCl and ethanol/acetone. After shelf-life, all the cultivars showed a decrease in firmness and an increase in the ratio SSC/TA. The h° parameter of the peel background color had a good correlation with firmness, SSC, TA and the ratio SSC/TA in some of the cultivars, but no relationships were found in the white-fleshed varieties and in two of the nectarines evaluated. The trend of the carotenoids content after postharvest ripening was found to be cultivar-dependent, while TAA or TPC showed an increase in nectarines and remained unchanged in peaches. The ethanol/acetone mix was able to extract almost the double of antioxidant compounds with respect to the ethanol/HCl extract.