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This investigation includes the hydrolysis kinetics, pH-solubility behavior, and pKa determinations for several model enol compounds. The in vitro transport properties of the enol compounds were investigated in model membrane systems. The apparent pH-solubility profiles of these compounds suggest the existence of cationic, neutral, and anionic forms in solution. The cationic form is unambiguously attributed to protonation of a nitrogen on a side chain. Although the anionic form was not expected, the characteristics of the pH-hydrolysis rate profile further support the existence of two pH-dependent equilibria in aqueous solution. Spectrophotometric pKa determinations confirmed the existence of two pKa values, in good agreement with solubility and hydrolysis data for the model compounds. The assigned charges were supported by media dielectric constant effects on the apparent pKa values. Baseline transport data indicate that only the neutral species of these compounds readily permeate hydrophobic membranes. However, in hydrated or porous membranes, both neutral and ionic species permeate the model membranes. Stability the compounds during the diffusion process was evaluated.