The lattice fragmentation and the recombination models of molecular-ion formation during sputtering are tested on oxygenated Nb and V surfaces. Submonolayers of Li atoms are used as probes to interact with the particles emitted under ultrahigh vacuum, low-energy bombardment (500 eV), static-mode sputtering conditions. Only the emission of positive Nb oxide ions from low-oxygen-coverage surfaces show consistency with the recombination model, similar to the recently reported Ti oxide ion results. The emission of positive V oxide ions and metallic dimers V2+ and Nb2+ deviate from the recombination model. All the negative-ion species show strong evidence of exclusive formation by fragmentation. An analysis of the energetics involved in the recombination processes suggests that negative molecular ions formed from nonadjacent atoms tend to be neutralized by electron detachment, leaving the fragmentation of the surface lattice the most probable way to produce stable negative molecular ions. © 1981 The American Physical Society.