This study evaluated the effect of thermal processing on the antioxidant potential, vitamin C, and total phenolic content of various parts, including edible co-products, of several Brassica vegetables. Overall, no significant differences were observed in the lightness of the samples after thermal processing, although the greenness and h0 values of the samples were affected (p < 0.05). Similar profiles were observed for the leaves, inflorescences, and stalks of the studied crucifers. The stalks of some varieties, including broccoli cv. Parthenon and kale cv. Crispa, showed higher vitamin C contents compared to that of their inflorescences (p < 0.05). Both steaming and sous-vide processing significantly reduced the vitamin C and total phenolic content of the crucifers studied (p < 0.05). The results demonstrated that Brassica co-products contain valuable and health-promoting substances that can be lost during thermal processing; this must be considered when calculating the dietary intake of these compounds from cooked vegetables.