Photoionization mass spectrometry reveals details of the multistep unimolecular mechanism, whereby the 2,2,6,6-tetramethyl-3,5-heptanedionato (thd−) anionic ligand decomposes, while still bound in the metal complex, to yield a gas-phase metal oxide product in metal−organic chemical vapor deposition (MOCVD) of lanthanide oxides from Ln(thd)3 precursors. The decomposition occurs with stepwise elimination of small closed-shell hydrocarbon fragments and carbon monoxide up to a penultimate Ln(OC2H) ethyneoxide, from which both LnO (dominant) and LnC2 (minor) products are derived. Formation of the metal oxide and carbide occurs in competition with a previously described mechanism − wherein sequential dissociation of ligand radicals produces the reduced metal Ln0. Evidence for gas-phase formation of a Ln2(thd)6 dimer as a result of expansion-cooling in the precursor source assembly is also given. Laser-assisted MOCVD of Eu(thd)3 on silica, with subsequent exposure to atmosphere, produces amorphous Eu2O3 with small areas of crystallinity attributed to reaction of the oxide with atmospheric carbon dioxide and water.