The gas-phase acidities (GA) of three cyclic ureas, imidazolidin-2-one (1H), benzoimidazolinone (2H) and parabanic acid (3H), have been experimentally determined. GA of 1H (1485.1 ± 8.4 kJ mol−1) was derived by equilibrium method using FT-ICR mass-spectrometer, while GAs of 2H (1413.7 ± 9.0 kJ mol−1) and 3H (1334.1 ± 9.0 kJ mol−1) were derived by the extended Cooks kinetic method (EKM) using ESI-TQ mass spectrometer. Quantum chemical calculations, at the Gaussian-n levels of theory (G3 and G4), shed light on structural and electronic effects on the intrinsic acidity of the studied compounds. These calculations confirmed the excellent consistency of our experimental results.