Abstract Comparison of the mass spectra of isotopically normal and 10B-enriched trimethylboroxines makes possible the identification of the various fragments. The spectra furnish independent substantiation of the molecular weight and structure of (CH3)3B3O3—a ring with considerable aromatic character. The 10 B 11 B isotopic abundance ratio determined from the principal-peak region is 0·251. Theoretical considerations involving molecular vibrations are introduced to show that there are minimum isotopic effects on fragmentation, and consequently on the calculated abundance ration, when bond rupture is produced by vibration of light atoms or groups against heavy boron-containing groups.