Transition metal oxide cations of the form MnOm+ (M = Y, La) are produced by laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. Cluster oxides for each value of n form only a limited number of stoichiometries; MO(M2O3)(x)(+) species are particularly intense. Cluster cations are mass selected and photodissociated using the third harmonic (355 nm) of a Nd:YAG laser. Multiphoton excitation is required to dissociate these clusters because of their strong bonding. Yttrium and lanthanum oxides exhibit different dissociation channels, but some common trends can be identified. Larger clusters for both metals undergo fission to make certain stable cation clusters, especially MO(M2O3)(x)(+) species. Specific cations are identified to be especially stable because of their repeated production in the decomposition of larger clusters. These include M3O4+, M5O7+, M7O10+, and M9O13+, along with Y6O8+. Density functional theory calculations were performed to investigate the relative stabilities and structures of these systems.