Two polyoxometalate Keggin-type anions, alpha-PM12O403- (M=Mo, W), were transferred to the gas phase by electrospray; their electronic structure and stability were probed by photoelectron spectroscopy. These triply charged anions were found to be highly stable in the gas phase with large adiabatic electron detachment energies of 1.7 and 2.1 eV for M=Mo and W, respectively. The magnitude of the repulsive Coulomb barrier was measured as similar to 3.4 eV for both anions, providing an experimental estimate for the intramolecular Coulomb repulsion present in these highly charged anions. Density functional theory calculations were carried out and compared with the experimental data, providing insight into the electronic structure and valence molecular orbitals of the two Keggin anions. The calculations indicated that the highest occupied molecular orbital and other frontier orbitals for PM12O403-are localized primarily on the mu(2)-oxo bridging ligands of the polyoxometalate framework, consistent with the reactivity on the mu(2)-oxo sites observed in solution. It was shown that the HOMO of PW12O403- is stabilized relative to that of PMo12O403- by similar to 0.35 eV. The experimental adiabatic electron detachment energies of PM12O403- (i.e., the electron affinities of PM12O402-) are combined with recent calculations on the proton affinity of PM12O403- to yield O-H bond dissociation energies in PM12O39(OH)(2-) as similar to 5.1 eV.