Gas-phase acidities (DeltaHdegrees(acid)) of lactones with ring sizes from four to seven have been measured on a Fourier transform ion cyclotron resonance mass spectrometer. Electron affinities (EAs) of the corresponding lactone enolate radicals were measured on a continuous-wave ion cyclotron resonance mass spectrometer, and the bond dissociation energies (BDEs) of the a C-H bonds were derived. In order of increasing ring size, DeltaHdegrees(acid) = 368.7 +/- 2., 369.4 +/- 2.2, 367.3 +/-2.2, and 368.3 +/- 2.2 kcal/mol and BIDE = 99.4 +/- 2.3, 94.8 +/- 2.3, 89.2 +/- 2.3, and 92.8 +/- 2.4 kcal/mol for beta-propiolactone, gamma-butyrolactone, delta-valerolactone, and epsilon-caprolactone, respectively, For their corresponding enolate radicals, EA = 44.1 +/- 0.3, 38.8 +/- 0.3, 35.3 +/-0.3, and 37.9 +/- 0.6 kcal/mol. All of these lactones are considerably more acidic than methyl acetate, consistent with a dipole repulsion model. Both BDEs and EAs show a strong dependence on ring size, whereas DeltaHdegrees(acid) does not. These findings are discussed, taking into account differential electronic effects and differential strain between the reactant and product species in each reaction.