The proton affinity (PA) of a molecule in the gas phase is an expression of its fundamental basicity and is a possible factor controlling the course of many ion-molecule reactions. The formation of an intramolecular hydrogen bond increases the PA value over that of similarly sized monofunctional molecules and the values of Delta S degrees are an indication of an intramolecular cyclization which occurs via hydrogen bonding in protonated bifunctional molecules. The first step in exploring these thermochemical properties has been the examination of the experimental proton-transfer equilibria using both ion cyclotron resonance (ICR) and high-pressure mass spectrometric (HPMS) studies. Parallel ab initio molecular orbital (MO) calculations on the protonated species show that the cyclized structures are the most stable species, in agreement with the experimental PA and entropy observations.