Abstract The thermophysical parameters, viz. excess molar volume V E, ultrasonic speed u, viscosity η and spectroscopic parameters, viz. IR and 1H 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 order: 2,3-BTD < 1,2-BTD < 1,3-BTD < 1,4-BTD. The partial and apparent molar quantities Q i E , isentropic compressibility K S E , excess viscosity η E and excess Gibbs free energies of activation for viscous flow Δ a G ∗E have also been estimated and analyzed. The spectroscopic measurements (IR and 1H 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.