Amphiphilic flavinium salt [10-dodecyl-5-ethyl-3,7,8-trimethylisoalloxazinium perchlorate (1b)], solubilized in micelles of sodium dodecyl sulfate (SDS), hexadecyltrimethyldmmonium chloride (CTAC), hexadecyltrimethylammonium nitrate (CTANO(3)) or Brij 35, catalyzes the chemoselective oxidation of thioanisole (2) to its corresponding sulfoxide 4. The sulfoxidation rates observed in micelles in the presence of 1b exceeded, in most instances, those of the reactions catalyzed by its hydrophilic homologue [5-ethyl-3,7,8,10-tetramethylisoalloxazinium perchlorate (1a)] in homogeneous solutions. Reaction rates were strongly dependent on the type of micellar matrix and on the pH value. The highest acceleration rate was found in SDS micelles at pH 4.4 (TOF = 3 x 10(3) h(-1)). In our experiments, micelles favoured the catalytic process rather than the non-catalyzed reaction, this effect being especially pronounced in cationic micelles, for which the non-catalyzed reaction was markedly suppressed. The ratio upsilon(cat)/upsilon(0) of the catalyzed and non-catalyzed reaction rates was almost 40 in CTANO(3), while in homogeneous solution its value did not exceed 7. (c) 2007 Elsevier B.V. All rights reserved.