The gas-phase basicity (GB), proton affinity (PA), and protonation entropy (Delta S-p degrees(M) = S degrees(MH+) - S degrees(M)) of arginine (Arg) have been experimentally determined by the extended kinetic method using an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. This method provides GB(Arg) = 1004.3 +/- 2.2 (4.9) kJ.mol(-1) (indicated errors are standard deviations, and in parentheses, 95% confidence limits are given). Consideration of previous experimental data using a fast atom bombardment ionization tandem sector mass spectrometer slightly modifies these estimates since GB(Arg) = 1005.9 +/- 3.1 (6.6) kJ.mol(-1). Lower limits of the proton affinity, PA(Arg) = 1046 +/- 4 (7) kJ.mol(-1), and of the "protonation entropy", Delta S-p degrees(Arg) = S degrees(ArgH(+)) - S degrees(Arg) = -27 +/- 7 (15) J.mol(-1).K-1, are also provided by the experiments. Theoretical calculations conducted at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of ca. 1053 kJ.mol(-1) after consideration of isodesmic proton-transfer reactions with guanidine as the reference base. Computations including explicit treatment of hindered rotations and mixing of conformers confirm that a noticeable entropy loss does occur upon protonation, which leads to a theoretical Delta S-p degrees(Arg) term of ca. -45 J.mol(-1).K-1. The following evaluated thermochemical parameter values are proposed: GB(Arg) = 1005 +/- 3 kJ.mol(-1); PA(Arg) = 1051 +/- 5 kJ.mol(-1), and Delta S-p degrees(Arg) = -45 +/- 12 J.mol(-1).K-1.