Recently, there has been considerable interest in hydrophilic/hydrophobic patterned surfaces because they serve as important templates for the selective deposition of various materials. We report a novel and simple method for the creation of hydrophilic/hydrophobic patterned surfaces using soft UV irradiation (365-nm wavelength). The method employs a photoinitiated hydrosilylation reaction of vinyl-terminated polydimethylsiloxane with H-Si groups catalyzed by platinum(II) acetylacetonate. In UV-irradiated regions, the photohydrosilylation reaction occurs, forming hydrophobic regions. In unirradiated regions, the remaining H-Si groups are converted into HO-Si groups in the presence of aqueous sodium hydroxide to form hydrophilic regions. The photoinitiated hydrosilylation reaction is completed within a little over 1 min, and this has been confirmed by reflection-absorption spectroscopy. The value of the water contact angle for the hydrophilic regions is about 10 degrees, and that for the hydrophobic regions is about 103 degrees. The success of pattern formation at the micrometer scale has been confirmed by scanning electron microscopy. The difference in the chemical structure at the surface has been confirmed by the decoration of the hydrophilic regions by a fluorescent dye and characterization with a fluorescence analyzer. Atomic force microscopy has shown that the height of the hydrophobic regions is about 20 nm. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 910-916, 2009