Ultrasonic speed u and dilatometric measurements of excess volume V-E have been carried out for mixtures of pyrrolidin-2-one (PY) with acetonitrile (ACN), propionitrile (PN), butyronitrile (BN), valeronitrile (VN), acrylonitrile (ACRN) and benzonitrile (BEN) at 303.15 K over the entire composition range. All the mixtures exhibit negative excess volumes, the curves being nearly symmetrical. The magnitude of the minimum increases steadily with decreasing molar size of the nitrile. The molar isentropic compressibility K-S has been computed by combining ultrasonic velocity and excess volume data. Excess molar isentropic compressibility K-S(E) is also negative in all the mixtures over the whole composition range. The data are interpreted in terms of moderate interactions of all kinds decreasing in importance with increasing nitrile carbon chain length. Partial and apparent molar quantities have been estimated to gain insight into interactions. Prigogine-Flory-Patterson theory has been utilised for the quantitative estimation of different contributions viz. interactional, free volume and P* effect to excess volume.