The adsorption and photodecomposition of Mo(CO)(6) adsorbed on Si(111) 7X7 surfaces has been studied with Auger electron spectroscopy, temperature programmed desorption, low energy electron diffraction and infrared reflection absorption spectroscopy in a single external reflection configuration. The external-reflection technique is demonstrated to have adequate sensitivity to characterize submonolayer coverages of photogenerated Mo(CO)(x) fragments. It is proposed that the first layer of Mo(CO)(6) adsorbs in ordered islands with a Mo(CO)(6) atop each adatom of the 7X7 reconstructed Si surface. UV irradiation of these islands produces a carbonyl fragment, identified as chemisorbed Mo(CO)(5). The Mo(CO)(5) thermally decarbonylates via two subcarbonyl intermediates with little CO dissociation. Photolysis of thicker layers results in the formation of Mo-x(CO)(y) dimers/polymers, as evidenced by the appearance of bridging CO, which is attributed to a facile association reaction. The dimer/polymer species correlate with deposition of C and O on the surface.