We have obtained direct mass spectrometric evidence that fullerene ions C-60(z+)(z = 1, 2, or 3) and C-58(z+) (z=1,2) undergo unimolecular dissociation by sequential emission of two C-2 units, on a time scale of 10(-5) s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C-56(+) from the C-60(+) parent ion within a given observational time window is dominated by successive loss of C-2; direct C-4 loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C-2 vs C-4 loss. (C) 1997 American Institute of Physics.