The adiabatic ionization potentials of TiO, ZrO, NbO, and MoO have been measured using two-color photoionization efficiency (PIE) spectroscopy and mass-analyzed threshold ionization (MATI). From the sharp ionization thresholds in the PIE and MATI spectra the following ionization potentials were derived : IP(TiO)=6.8197(7) eV, IP(ZrO)=6.812(2) eV, IP(NbO)=7.154(1) eV, and IP(MoO)=7.4504(5) eV, These values have been combined with the ionization potentials of the metal atoms and the bond energies of the transition metal oxide cations, D-o(MO+) [M. R. Sievers et al,, J. Chem. Phys, 105, 6322 (1996)] to derive the bond energies, D-o(MO), of the neutral metal monoxides; D-o(TiO)=6.87(7)eV, D-o(ZrO)=7.94(11) eV, D-o(NbO)=7.53(11)eV, D-o(MO) = 5.44(4) eV. It is argued that these values are more accurate than the currently accepted values and hence are recommended for future work. Experimental evidence suggests that the ground state of MoO+ is the (4)Sigma(-) state arising from the delta(2)sigma(1) configuration.