To calculate the proton affinity of fullerene (C-60), density functional theory was used to determine the global minimum energy structures of both fullerene and its protonated forms. Vibrational frequency calculations were used to check the nature of these predicted structures. In the protonation of C-60 in the gas phase, the proton preferentially lies above the carbon atoms at a distance of 1.10 angstrom, which suggests a bond of covalent nature. The proton affinity for fullerene was calculated as 201.8 kcal/mol, compared with the experimental value between 204 and 207 kcal/mol obtained by proton-transfer bracketing studies using Fourier transform mass spectrometry. All five transition states for intramolecular proton transfer in fullerene were found, three for the first time. The activation energy (E-a) barriers for proton migration were calculated and ranged from 27 to 90 kcal/mol. Different functional groups attached to fullerenes, and their influence on E-a values are discussed, as are all the possible proton transfers for nonfunctionalized fullerenes.