Here, cryogenic ion mobility-mass spectrometry (cryo-IM-MS) is used to investigate intracluster proton transfer reactions of 4-aminobenzoic acid during the transition from solution to the gas phase. Previous studies have shown that protonation of the amine group of 4-aminobenzoic acid (4-ABAH(+)) is favored in solution (N-protomer), whereas protonation of the carboxylic acid group is favored in the gas phase (O-protomer). Results from cryo-IM-MS (80 K) studies of hydrated 4-ABAH(+) ions, 4-ABAH(+)(H2O)(n), are interpreted as evidence that the proton transfer reaction occurs through a water bridge at n = 6 connecting the -NH3+ and -COOH groups, that is, a Grotthuss mechanism. The weak binding energy of water molecules imposes limits for obtaining first-principles collisional cross sections (CCSs) of hydrated ions; consequently, candidate structures for 4-ABAH(+)(H2O)(0-6) ions are derived by correlating experimental arrival-time distributions to theoretically determined CCSs. To our knowledge, these are the first first-principles determinations of CCS for hydrated ions. A polar cosolvents, particularly acetonitrile, have been postulated to inhibit proton transfer by blocking the Grotthuss mechanism, but our data suggest that acetonitrile simply stabilizes the ammonium ion.