The mass spectra of indole alkaloids of the 21-oxoaspidoalbidine type show an unusual fragmentation pattern, giving an ion at m/e 160 as (usually) the base peak. Using the known alkaloid dichotamine and two new alkaloids of this group isolated from Aspidosperma exalatum, with deuterations and high-resolution mass spectral measurements, a rationale for this unprecedented fragmentation was developed. The first step, loss of CO2, pro-hibits the normal aspidospermine-type fragmentation and leads instead to ion fragments including C-20 and one or both aliphatic carbons of the dihydroindole ring. All major peaks in the spectra are assigned tentative origins and structures on the bases of their established molecular formulas and their shifts in deuterated materials. © 1966, American Chemical Society. All rights reserved.