Site-selective adsorption of ethylene on Si(001) has been investigated by means of scanning tunneling microscopy (STM). Two different adsorption pathways are identified with ethylene adsorbed on one and two dimers, respectively. Preadsorption of atomic hydrogen is found to increase the reactivity of the two-dimer pathway, which is the minority channel on the clean surface, by a factor of 45. The results are interpreted in the framework of a precursor mediated adsorption process with a conversion barrier between precursor and final adsorption state that can be efficiently controlled by changing the local electronic structure of the surface. (C) 2009 Elsevier B.V. All rights reserved.