The occurrence or absence of proton transfer from derivatized fullerene trications C 60 H 3+ , to the parent neutral XH, is employed to obtain upper or lower limits to the apparent gas-phase acidity GA app as well as to the estimated absolute gas-phase acidity. A comparison with the reactivity of analogous dicationic adducts indicates that C 60 XH 3+ is generally more acidic than C 60 XH 2+ : for example, the difference in GA values for C 60 NCCH 3 n+ (n = 2, 3; formed in the addition reaction of C 60 n+ with CH 3 CN) is estimated to be at least 42 kcal mol −1 . In almost all instances, estimated GA values for the tricationic adducts are far below the gas-phase basicities of the parent neutral XH, mirroring a trend seen from the proton-transfer reactivity of multiply protonated protein molecules: a qualitative difference between the fullerene adducts and the multiply protonated biomolecules routinely produced by electrospray ionization is that the fullerene adduct ions (with the exception of C 60 NH 3 3+ ) appear to possess only one acidic proton.