The dielectric response of 2-pyrrolidone + water binary mixtures was studied using microwave dielectric spectroscopy at 298.15 K and ambient pressure in the entire composition range. The mixture behaviour is described according to three Debye terms whose evolution with composition is analyzed. A complex mixture structure is inferred from the results arising from the ability of homo and hetero-association that show both molecules. The effect of 2-pyrrolidone on water structure at low lactam concentrations is very remarkable resulting in a reinforcement of the hydrogen bonding in the mixture, which is in agreement with previous thermophysical and theoretical studies. (C) 2007 Elsevier B.V. All rights reserved.