The gas phase proton affinity (PA) of guanidine has been determined by FT-ICR mass spectrometry. The value obtained (233 kcal/mol) is in good agreement with previous theoretical estimations. The intrinsic basicity as well as the activation barriers for the amino group rotation in both the neutral molecule and the protonated species are discussed at the G2 and G2(MP2) level of theory. At these levels of theory the experimental PA is fairly well reproduced (PA value 235.7 kcal/mol). Moreover, the use of DFT optimized structures (using BLYP and SWVN functionals) is considered in order to search for an economical alternative to obtain reliable thermodynamic data for larger systems. The agreement between the G2(MP2) calculations and the experimental values, increases slightly when the former are performed on DFT optimized structures (PA value 234.8 kcal/mol). The BLYP/6-311 +G(3df,2p) calculation gives the same value as the G2 and G2(MP2) ones. When using the SWVN functional, the experimental proton affinity is better reproduced (PA value 233.8 kcal/mol). Finally, in an attempt to understand the difference between guanidine basicities in solution and in the gas phase, the influence of the solvent is treated at the self-consistent reaction field level (SCRF) of theory.