A nearly mono-dispersed spherical electroactive particle, 1.16 mu m in diameter, was synthesized by coating polystyrene particles successively with polyallylamine and hemin with the aim of investigating the redox reactivity of large particles. Each particle contains 2 x 10(7) hemin moieties. Its suspension in 2-propanol solution was stable without aggregation. The voltammogranis showed a pair of redox waves due to the immobilized hemin. The cathodic current was controlled by diffusion of the particle toward the electrode. The amount of hemin participating in the electrode reaction was only a quarter of the amount of loaded hemin. Dioxygen in solution was reduced catalytically with the hemin-coated particles. The catalytic efficiency for the hemin-coated particles was higher than that for free hemin molecules, because dioxygen confined in particles as well as dissolved dioxygen can participate in the catalytic reaction. The catalytic reaction rate constant was evaluated on the assumption of a pseudo-first-order reaction to give 0.08 M-1 s(-1), which was half the rate constant of hemin. (c) 2005 Elsevier B.V. All rights reserved.