Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH3 (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH4 at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu2Si structure. These Cu2Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH3)(3)SiH desorbed near 200 K from CH3/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH3)(3)SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH3)(3)SiH at 200 and 420 K were believed to be due to the thermal reaction of CH3 with the low Si density and high Si density (i.e., Cu2Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu2Si phase covered the surface, only the 420 K peak was present during CH3/Si/Cu(100) TPD. Results also suggested that (CH3)Si and possibly some (CH3)(2)Si intermediates predominated on the surface below room temperature, and (CH3)(3)-Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH3)3SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).