The photoionization efficiency (PIE) spectra for M(CO)(n)(+) (n = 0-6) from M(CO)(6), M = Cr, Mo, and W, have been measured in the photon energy range of 650-1600 Angstrom. Based on the ionization energies for M(CO)(6) and appearance energies (AEs) for M(CO)(n)(+) (n = 0-5) determined here, we have obtained estimates for the sequential bond dissociation energies (D-0) for CO-M(CO)(n-1)(+) (n = 1-6). The comparison between the D-0 values for the Cr(CO)(6)(+) system obtained here and in the recent collisional induced dissociation and theoretical studies suggests that D-0 values for CO-M(CO)(n-1)(+) (n = 3-6) based on this PIE experiment are reliable. The PIE results reveal the general trend for individual D-0 values that D-0[CO-Cr(CO)(n-1)(+)] < D-0[CO-Mo(CO)(n-1)(+)] < D-0[CO-W(CO)(n-1)(+)] (n = 3-6). The comparison of the first D-0 values for M(CO)(6)(+) obtained here and those for M(CO)(6) reported previously provides strong support for the theoretical analysis that the importance of relativistic effects, which give rise to more efficient M to CO pi-back-donation in M(CO)(6), is in the order W(CO)(6) > Mo(Co)(6) > Cr(CO)(6) (C) 1997 American Institute of Physics.