The thermal and photoinduced desorption and decomposition of Fe(CO)(5) on clean and O-covered Ru(001) surfaces were studied. Adsorption of Fe(CO)(5) on Ru(001) is associated with a partial decomposition, resulting in the formation of CO and Fe(CO)(x) fragments. In the thermal desorption spectrum for mass 28, the surface-stabilized decomposition product gives rise to a peak at 270 K, whereas the first and second monolayers of molecular Fe(CO)(5) desorb at 190 and 160 K, respectively. The photochemical studies of Fe(CO)(5) at mono- and multilayered molecular coverages on Ru(001) were carried out by UV irradiation at various wavelengths (290-450 nm), Irradiation at wavelengths > 370 nm resulted in photodesorption, while photodecomposition showed significant contribution at shorter wavelengths. The total cross sections for the photochemical process closely follow the UV absorption spectrum of Fe(CO)(5) in the gaseous phase, suggesting that the photoreaction is mainly due to the direct absorption of UV photons by the adsorbed Fe(CO)(5) molecule. The photodecomposition yields reactive intermediates that subsequently form Fe-x(CO)(y) clusters. These species thermally decompose, desorbing the CO moieties and depositing Fe atoms on the surface. Dissociative adsorption of Fe(CO)(5) has also been observed on O-covered Ru(001). O adatoms create inhomogeneity in the adsorption sites for Fe(CO)(5), as seen from the broadened desorption peak of the first molecularly adsorbed layer of Fe(CO)(5). In contrast to the clean surface, irradiation of Fe(CO)(5) adsorbed on O/Ru(001) at 290 nm produced relatively lower yields of photodecomposition and a higher extent of photodesorption attributable to the more effective quenching of electronically excited Fe(CO)(5) by the O-covered surface.