Publisher Summary This chapter provides a large collection of solution data and a thoroughly updated discussion of thermodynamic, kinetic, and structural results. This chapter covers aromatic azoles exclusively—that is, aromatic five-membered rings containing only carbon and nitrogen atoms, and their benzo derivatives. This chapter also discusses parent structure on theoretical grounds. One of the oldest methods to get insight into molecular structure, the study of acid-base properties, is today in rapid development. This is particularly true for the azoles. Gas phase studies of the acidity and basicity of azoles provides information on lone pair interactions, transmission of substituent effects in five-membered rings, relationships between acidity and basicity, aromatic substituent effects of nitrogen-linked derivatives and thermodynamic and kinetic parameters for proton transfer between nitrogen atoms in polyazoles. Because of the biological significance of some azoles (pyrrole, indole, imidazole, benzimidazole) and the consequences of acid–base equilibria in their functions, a continuous interest in the behavior in water is to be expected. In the case of N-unsubstituted imidazoles and pyrazoles, complexation of the pyridine-like nitrogen causes a considerable increase in the acidity of the pyrrolelike nitrogen.