A wide range of N-substituted mono- and dihydroxamic acids undergo oxygen abstraction on reaction with V(III), V(IV), and Mo(V) compounds to form hydroxamates of V(V) and Mo(VI) respectively together with the corresponding amides and diamides. The molybdenyl and vanadyl hydroxamates form metal-oxygen clusters under FABMS conditions. The X-ray crystal structures of [MoO2{CH3(CH2)(n)C(O)N(C6H5)O}(2) (1 and 2) (n = 4, 5) show monomeric structures with structural trans effects and consequent weakening of the Mo-O(ligand) bonds which may account for the tendency to form clusters in FABMS. In constrast, the electrospray MS of the vanadyl dihydroxamates, VO(OH)[PhN(O)C(O)(CH2)(n)C(O)N(O)Ph] (n = 3, 5) and VO(OH)[p-CH3C6C4N(O)C(O) (CH2)(n)C(O)N(O)C6H4-CH3) (n = 2, 4) show the presence of dimers in solution.