Various Pt-carbonyl cluster complexes ([Pt-3(CO)(3)(mu - CO)(3)](n)(2-) (n = 3-8)) were prepared by simply bubbling CO through PtCl62- solutions with different solvents. The number of layered Pt-3(CO)(3)(mu-CO)(3) units (n) was independent of water content and the length of the alkyl chain of the solvent, but it increased with a decrease in the permittivity of the solvent. The Pt-carbonyl cluster complexes were applied to the preparation of Pt-nanoparticle-loaded carbon black (Pt/CB). The mean size of the resultant Pt nanoparticles increased with n, whereas their size distribution remained narrow (+/- 0.3 nm) irrespective of n. For the oxygen reduction reaction, the specific activity (SA) of each Pt/CB catalyst was almost equivalent to that of the Pt/CB catalyst produced by Tanaka Kikinzoku Kogyo (Pt/CB-TKK). Meanwhile, the mass activity (MA), the product of SA and real specific surface area (SSA(real)), at 0.9 V of each Pt/CB catalyst was superior to that of Pt/CB-TKK because the SSA(real) evaluated experimentally for the Pt/CB catalysts was higher than that for the Pt/CB-TKK. In particular, the MA at 0.9 V of the Pt/CB catalyst (Pt-et(50)/CB) prepared from [Pt-3(CO)(3)(mu - CO)(5)(2-) synthesized in ethanol-water (50:50 v/v) was about 1.7 and 2.0 times higher than those of commercial Pt/CB-TKK and E-TEK catalysts, respectively. (C) 2011 Elsevier B.V. All rights reserved.