Proton affinity, PA(Glu), and protonation entropy (i.e., the difference Delta S-p degrees(Glu) = S degrees(GluH(+)) - S degrees(Glu)) of glutamic acid have been experimentally determined by the extended kinetic method using electrospray ionization triple quadrupole-time-of-flight (ESI-Q-TOF) tandem mass spectrometry. The values deduced from these experiments are PA(Glu) = 945.3 +/- 2.8(5.8) kJ . mol(-1) and Delta S-p degrees(Glu) = -28 +/- 4(9) J . mol(-1) . K-1 thus leading to a gas-phase basicity, GB(Glu), of 904.4 +/-3.0(6.4) kJ . mol(-1) (uncertainties are standard deviation and, in parentheses, 95% confidence limit). Theoretical calculations performed at the G3MP2B3 level provide information on the structures, conformations, and energetics of the neutral and protonated species. Thermochemical data are calculated at this level and include a correction to the computation of the entropy associated with hindered rotation. When the. lowest energy conformers of protonated and neutral glutamic acid are considered the following values are calculated: PA(Glu) = 948.1 kJ . mol(-1) and Delta S-p degrees(Glu) = -31.3 J. mol(-1) . K-1. Using G3MP2B3 data to estimate the gas-phase distribution of conformers at 298 K, the averaged molar quantities becomes PA(Glu) = 949.8 kJ . mol(-1) and Delta S-p degrees(Glu) = -36.0 J . mol(-1) . K-1. Both computations give comparable GB(Glu) = 906.4-906.7 kJ . mol(-1).