A focused laser beam at a wavelength of 514 nm was used to pattern octadecylsiloxane monolayers grown on distinct substrates which expose a Si oxide surface. Patterning expts. on conventional Si substrates with a native oxide layer result in a decompn. of the monolayer along well-confined lines. The line widths generally are well below the focal laser spot diam. of 2.5 +- 0.3 mm indicating a nonlinear dependence of the overall process on the laser intensity. Very similar results were obtained using thermally oxidized Si substrates. But expts. to pattern coated quartz plates failed even if the laser power and hence the local irradiance at the surface was raised to much higher values. Altogether, these expts. strongly support a photothermal excitation mechanism, i.e., a decompn. of the monolayer as a result of the laser induced local temp. rise.