To identify a possible use for some agroindustrial wastes generated from the sugarcane industry, we evaluated the antioxidant capacity of B and C molasses and vinasses from the sugar and bioethanol production processes. Molasses and vinasses were characterized by physicochemical methods. Subsequently, the samples were diluted in distilled water at five concentrations to obtain aqueous extracts. Total phenolic content (TPC) of the samples was determined using a spectrophotometric method and was expressed in mg equivalents of gallic acid. The antioxidant capacity of each sample was determined by DPPH (2,2-diphenyl-1-picrylhydrazyl radical) and ABTS (2,2-azino-bis-[3-ethylbenzothiazoline-6-sulfonic acid]) methods, as well as the ferric-reducing power in FRAP (Ferric-reducing Antioxidant Power) assay. We found that, both for TPC and antioxidant capacity, the vinasses showed significantly higher values than the B and C molasses. These results showed a strong correlation between TPC and antioxidant capacity and revealed a remarkable increase in TPC and total antioxidant agents present in the extracts throughout the sugarcane transformation process. These findings allowed identifying vinasses as the by-product with the best antioxidant properties. Our work constitutes a first step in the study of molasses and vinasses as a promising antioxidant agent and as a novel resource to test in proliferative trials in cellular systems in vitro.