Urea undergoes O-protonation in the gas phase to yield a product that is thermodynamically more stable than the N-protonated isomer, as also is the case in aqueous solution. The proton affinity and gas-phase basicity of urea, determined by using the kinetic method, are 873.5 +/- 5.0 and 841.6 +/- 5.0 kJ/mol, respectively. These values are in excellent agreement with G2(MP2) calculations, which give PA = 872.4 kJ/mol. The entropy requirements for the competitive dissociation channels of the proton-bound heterodimers of urea and the chosen reference compounds are measured and lead to the conclusion that urea and these references have almost equal protonation entropies (Delta(Delta S degrees) = 0.80 J/kmol). In comparison with proton affinities of acetone and acetamide, the proton affinity of urea is understandably enhanced by resonance stabilization in both neutral and protonated urea, and an upper limit to the PA value is established by a resonance saturation effect. These considerations provide a basis for the explanation of some aspects of the reactivity of urea.