Rapid patterning of octadecylsiloxane monolayers at ambient conditions is demonstrated using a focused laser beam at a wavelength of 514 nm. Surface-oxidized silicon substrates have been coated and subsequently processed at distinct laser powers over a wide range of writing speeds up to 25 mm/s. The method allows for a well-confined local decompn. of the monolayer with an unexpectedly high lateral resoln. which is significantly below the diffraction limited laser spot diam. of the optical setup. In particular, at a 1/e2 focal spot diam. of about 2.5 mm line widths close to 200 nm are reached. Complementary expts. at a spot diam. of about 1.2 mm yielded irregular lines with a min. width close to 100 nm. The underlying highly superlinear dependence of the patterning process on the laser intensity is attributed to the interplay between the laser-induced local temp. rise and the thermally activated decompn. of the org. coating. A simple thermokinetic anal. of the data allows one to est. effective kinetic parameters of the decompn. process and reproduce the exptl. obsd. functional dependence of the line width on the incident laser power and the writing speed.