The kinetics of radical-scavenging activities for colchicine and tropolone remain unknown. Their antioxidant activities were determined by the induction period (IP) method in the polymerization of methyl methacrylate initiated by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN, R*) or benzoyl peroxide (BPO, PhCOO*) using differential scanning calorimetry (DSC) under nearly anaerobic conditions. The IPs for colchicine and tropolone were very short despite the addition of a high concentration of these compounds relative to initiators; the stoichiometric factor (n, the number of moles of PhCOO* trapped by the antioxidant) was approximately 0.03 and 0.04 for colchicine and tropolone, respectively. The n value of these compounds for R* was less than that for PhCOO*. The rate constant of inhibition to that of propagation (kinh/kp) for both compounds was 23-27, and the difference between them was considerably small. Both compounds had weak antioxidant properties at very high concentrations.