The direct formation of hydrogen peroxide from H-2 and O-2 was successfully carried out in a capillary microreactor at room temperature and atmospheric pressure. A key element in sustaining the activity of the catalyst is the incarceration of the palladium nanoparticles in a cross-linkable amphiphilic polystyrene-based polymer, prepared following the protocol of Kobayashi [R. Akiyama, S. Kobayashi, J. Am. Chem. Soc. 125 (2003) 3412-3413]. The immobilization effectively reduced the leaching of palladium under acidic conditions. Applying the catalyst as a coating, on the inner walls of a capillary enabled the sustained production of 1.1% hydrogen peroxide over at least 11 days. The highest catalyst utilization in a 2 mm capillary reactor was 0.54 mol(H2O2)/h g(Pd). When the inner diameter of the reactor capillary was reduced to 530 mu m, the rate was enhanced fourfold to 2.28 mol(H2O2)/h g(Pd) corresponding to a turnover frequency of 0.067 s(-1). (C) 2009 Elsevier Inc. All rights reserved.