pK(a) values of benzophenone ketyl and benzil ketyl radicals were determined as 9.4 and 12.4, respectively. We can successfully explain the difference in quantum yield of the proton transfer between benzophenone ketyl and benzil ketyl radicals by these values. Reaction enthalpies of the proton transfer are the same (-80 kJ mol(-1)) for these radicals, and the difference in pK(a) value can be explained by that reaction entropies. Reaction entropies between two radicals are discussed by the possible structure of the radicals. (c) 2005 Elsevier B.V. All rights reserved.