The thermophysical parameters, viz. excess molar volume V-E, ultrasonic speed mu, viscosity eta and spectroscopic parameters, viz. IR and H-1 NMR have been measured for the mixtures of pyrrolidin-2-one and isomeric butanediol (1,2-, 1,3-, 1,4- and 2,3-BTD) over the whole composition range at 308.15 K. Results indicate that intermolecular interactions for different mixtures increase in the isentropic compressibility K-S(E) order: 2,3-BTD < 1,2-BTD < 1,3-BTD < 1,4-BTD. The partial and apparent molar quantities Q(i)(E), excess viscosity eta(E) and excess Gibbs free energies of activation for viscous flow Delta(a)G(*E) have also been estimated and analyzed. The spectroscopic measurements (IR and H-1 NMR) confirm that the interaction between unlike molecules takes place through hydroxyl groups of isomeric butanediol. A good agreement is obtained between excess quantities and spectroscopic data. (c) 2004 Elsevier Ltd. All rights reserved.