Ionization efficiency curves of both singly and doubly charged fullerene ions (C-60-2n(m+), 1-5, m = 1 and 2) were determined for the first time, employing electron impact ionization. Appearance energies of all these fragment ions were derived from these data. Relative abundances of the fragment ions were computed from the measured ion intensities and compared with those available in the literature. The results of the present investigation are compared with the data available in the literature on multiply charged and fragment ions. Based on the measured appearance energies of the fragment ions, ionization energies for the formation of doubly charged fragment ions are derived. Enthalpies of formation of C-60-2n (fragment) fullerenes are derived for the first time. The enthalpy data of fullerenes derived in the present work support the loss of C-2n as the mechanism of fragmentation.