The satellite structure in the valence shell x-ray photoelectron spectrum (XPS) of CO is studied. The configuration interaction method is used to construct correlated many-electron wave functions for the different final states of CO+. It is found that (1) the conventional shake-up model cannot explain the satellite structure; (2) a considerable amount of intensity is lost from the main, "single-hole," î+2 states (3σ-1, 4σ-1, and 5σ-1) due to electron correlation; and (3) the 3σ-1 peak satellites are at lower, rather than higher, binding energy than the main peak. Franck-Condon broadening is estimated and shown to be a major cause of the large width of the observed structures on the high-binding-energy side of the 4σ-1 peak. Our conclusions are supported by the results of other electron spectroscopies in addition to XPS. © 1977 The American Physical Society.