The dielectric response of 2-pyrrolidone+water binary mixtures was studied using microwave dielectric spectroscopy at 298.15K 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 heteroassociation 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.