In this work, the critical role of temperature (the values assayed were 22, 37 and 60 °C), the chemical characteristics of the medium (ethanol, methanol and water) and the reaction time on the antiradical capacity of phenolic systems was studied. An initial increase and a following decrease in antiradical activity were observed for catechin and resveratrol solutions in all solvents assayed. The maximum antioxidant activity was higher and occurred in a shorter time as the storage temperature was increased or the solvent polarity was decreased. The maximum values of the latter variable in the catechin and resveratrol cases (reaching an antiradical activity value higher than 50% in comparison with the initial one) were detected when oxidation was carried out in ethanol at 60 °C after 6 and 24 h of storage, respectively. Such variations were due to different reaction pathways. In fact, oxidative polymerization and oxidative formation of hydroxyl groups were found to be responsible for the enhancements of antiradical activity in catechin and resveratrol, respectively. A similar trend with variations of temperature in the different media was also observed in the grape extract case. The evolution of antiradical activity was followed by high-performance liquid chromatography analysis, which pointed to the phenol polymerization of the monophenols detected (catechin, gallic acid, epicatechin and quercetin) as mainly responsible for such variations.