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