The mass spectra of a number of naturally occurring, as well as recently synthesized, oxindole alkaloids have been investigated. By the use of deuterium labeling, as well as variations in substitution in the aromatic and alicyclic portions of the molecule, the principal mass spectral fragmentation processes have been elucidated. Some of these processes are uniquely different from those observed with other indole alkaloids, and mass spectrometry can thus be used as an excellent criterion for establishing membership in this class of alkaloids. As an illustration, there is reported the first isolation of a new oxindole alkaloid, carapanaubine, from an Aspidosperma species—mass spectrometry pointing toward an oxindole structure, which was subsequently confirmed by synthesis. © 1963, American Chemical Society. All rights reserved.