A highly focused laser beam at a wavelength of 514 nm is used to pattern hydrophilic, surface-oxidized silicon substrates. Irradn. converts the affected surface areas from hydrophilic to hydrophobic and hence provides a means to prep. hydrophilic/hydrophobic patterns. The patterns are stable for weeks or longer, even if the samples are stored at ambient conditions. Contrary to ordinary avenues to hydrophilic/hydrophobic patterns no coating is required. This makes the procedure extremely simple and robust. Routine patterning can be carried out over large areas at fast writing speeds under ambient conditions. At a focal spot diam. of about 2.5 mm, hydrophobic lines with a width down to 0.4 mm are prepd. indicating a highly superlinear dependence of the overall process on the laser intensity. The patterned substrates might be useful in a broad variety of applications. In particular, as shown here, they represent suitable templates for the directed self-assembly of org. monolayers.