Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at lambda = 514 nm and a l/e(2) spot diameter of 2.6 mu m results in local decomposition or the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for sub:wavelength patterning. with line widths between 0.4 and 1.4 mu m. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 rim are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g. for hybrid microelectronic and sensor applications.