Experimental high-resolution core photoelectron C1s and O1s shake-up spectra of molybdenum hexacarbonyl, Mo(CO)(6), are reported and compared with results of semiempirical INDO/CI calculations. Several hitherto unobserved peaks are identified and assigned. It is found that an intra- and inter-group classification can be used to describe the transitions. The transitions of lowest energy have Mo-CO inter-group character. Dynamical effects on the line widths of the experimental shake-up peaks are discussed. A discussion of the relevance of experimental results obtained for the Mo(CO)(6)e for CO adsorption on metal surfaces is presented.