Complex formation in ternary liquid mixtures of heterocyclic compounds, viz. pyridine and quinoline with phenol in benzene has been studied through ultrasonic velocity measurements (at 2 MHz) in the concentration range of 0.010-0.090 at varying temperatures of 35, 45 and 55degreesC. The ultrasonic velocity and density data are used to estimate adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility and specific acoustic impedance. These acoustical parameters, in turn, are used to study the solute-solute interactions in these systems. The ultrasonic velocity shows a maxima and adiabatic compressibility a corresponding minima as a function of concentration for these mixtures. The results indicate the possible occurrence of complex formation between unlike molecules through intermolecular hydrogen bonding between the nitrogen atom of pyridine and quinoline molecules and the hydrogen atom of phenol molecule. Further, the excess values of adiabatic compressibility and intermolecular free length have also been evaluated and discussed in relation to complex formation. The infrared spectra of both the systems, pyridine-phenol and quinoline-phenol, have been also recorded for various concentrations at room temperature (35degreesC) and found to be useful for understanding the presence of N...HO bond complexes and the strength of molecular association at specific concentrations. (C) 2003 Elsevier B.V. All rights reserved.