The mixtures of phenol and some heterocyclic bases of biological importance have been studied by i.r. spectroscopy in CCl4 and equilibrium constants (K) and thermodynamic data (ΔH, ΔG, ΔS) have been determined at 25 and 40 °C. The formation constants of hydrogen-bridged phenol complexes were found to be related to the protonation constants of the bases, but no strict linearity can be proved in many cases. The participation of the non-bonded electron pair of the heteroatom in the π-system of the rings plays a dominating role in the proton-acceptor processes.