o-Methoxyphenols are antioxidants widely used in the cosmetic and food industries. Dimers from 1, 2, or 3 were synthesized and their radical-scavenging and biological activities were compared with those of the original or other phenols. Radical-scavenging was evaluated from a kinetic induction period method (IPM). To simulate biomimetic thiolcooxidation with antioxidants, the behavior of mixtures of 1, 2, 3, 4, or catechin with mercaptomethylimidazole (MMI), a thiol was investigated using IPM. Polyphenols 4 and catechin was accompanied by extensive oxygen uptake, suggesting the formation of thiyl radicals from MMI and their reaction with molecular oxygen. In contrast,1 markedly enhanced radical-scavenging without oxygen uptake, probably because of the formation of EUGQM/MMI-conjugates. 2 showed relatively small oxygen uptake, probably resulting from the predominant formation of benzyl radicals. Intracellular reactive oxygen species (ROS) in cancer cells by 4 ,but not by compounds 1, 2, 6, 7, 8, 9, and 10 was found, suggesting a possible link between physicochemical oxygen-uptake and intracellular ROS. The induction of apoptosis by 4 in HL-60 cells was accompanied by intracellular ROS. Dimers 6 and 7 inhibited nuclear factor (NF)-kappaB, activation stimulated by lipopolysaccharide (LPS) in RAW 264.7 cells. Also,6 ,7,and 9 inhibited LPS-induced cyclooxygenase-2 expression in RAW 264.7 cells in a dose-dependent manner, whereas 1, 2, and 3 did not. Dimerization of o-methoxyphenols may be a useful tool for the design of drugs to act as potent chemopreventive and anticancer agents.