Catechins show various biological activities, for which beneficial as well as adverse effects have been observed. Unfavorable properties of catechins are often attributed to reactive oxygen species (ROS) production. Our aim was to investigate whether catechins affect protein via ROS generation. We examined the effects on bovine serum albumin (BSA) of (+)-catechin with Cu²⁺ as well as ascorbate with Cu²⁺. The ascorbate/Cu²⁺ system is a metal-catalyzed oxidation (MCO) system and can produce ROS. Both systems altered spectroscopic properties of albumin, increased protein carbonyl content and resulted in several conformational changes. Catalase and hydroxyl radical scavengers markedly suppressed protein modifications by the ascorbate/Cu²⁺ systembut not those by the (+)-catechin/Cu²⁺ system. As far as we observed, no protein alteration induced by the (+)-catechin/Cu²⁺ system correlated with ROS formation.