The electronic and structural properties of monohafnium oxide clusters, HfOn- and HfOn (n = 1-6), are investigated using anion photoelectron spectroscopy and density-functional theory at the B3LYP level. The observed ground-state adiabatic detachment energy is low for HfO- (0.5 +/- 0.1 eV) and HfO2- (2.125 +/- 0.010 eV), roughly constant for HfO3- (3.6 +/- 0.1 eV), HfO4- (3.67 +/- 0.05 eV), and HfO5- (3.9 +/- 0.1 eV), and substantially higher for HfO6- (4.9 +/- 0.1 eV). Activated oxygen species, such as radical, superoxide, peroxide, diradical, and triradical, are identified in the HfOn- and HfOn, clusters. The Hf center is shown to be flexible to accommodate the oxygen species. The sum of formal Hf-O bond orders around the Hf center is equal to four for all of the neutral clusters studied, and five for all of the anions. The O-rich HfOn- and HfOn (n = 3-6) clusters provide well-defined molecular models to understand O-2 adsorption and activation on an Hf center.