High temperature gaseous reaction equilibria involving GdO, HoO, ErO, TmO, LuO, and certain reference oxides have been studied by mass spectrometry, using a molecular effusion beam source. From the reaction thermochemistry, the dissociation energies, D00, were derived as 169.5±3 kcal/mol (GdO), 144.1±3 kcal/mol (HoO), 143.9±3 kcal/mol (ErO), 121.8±3 kcal/mol (TmO), and 159.4±2 kcal/mol (LuO). Some of these values differ substantially from previous determinations. However, the new results together with several recent determinations and re‐evaluations yield a consistent set of results for the entire series from LaO to LuO. Trends in D00 values across the series can be correlated remarkably well with changes in the 4fn6s2→4fn−15d6s2 electronic promotion energies in the gaseous metal atoms. Various aspects of the bonding are discussed.

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