The mass spectra of live sets of stereoisomers possessing an eburnnne basic skeleton (vincamine, vlncrrrrmu * 'c acid ethyl ester, deem' ' e, deoxyvincaminic acid ethyl ester, apovincaminic acid ethyl ester and all their possible epimers) have been compared. All of the epimers, among them the 3H-f3 isomers, not studied formerly, have characterMcaUy Merent electron impact spectra. For vincamine and its epimers similar Merences were also f o d in the argon charge exchange ionization, mass-adysed ion kinetic energy and fdlisionally activated mass-adysed ion Linetie energy spectra. me main effects of stereoisomem on the abundance ratios dearly indicated the occurrence of some highly stere~~peci6c or stereaselective decomposition proeessea. Condusbns concerning the mechanism of these reactiom, based on the normal and metestable ion spectra as well as on some energetic data for the epimers, have also been drawn. For several fragment ions the observed unexpeded abundance ditlerencea seem to be rationalized in terms of the competing effects of other decomposition processes of higher stereuselectiVity.