The guanidinium protonation constant pK(a2) of L-canavanine has been determined in aqueous solution at different temperatures and different ionic strengths, using a potentiometry technique. The effect of an organic solvent added to the aqueous solution on the protonation processes is also reported and explained. The organic solvents used were methanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetone, and dioxane. The pK(a2) values for the ionization in a water (1) + dioxane (2) mixture have been determined at four different temperatures from T = (298.15 to 328.15) K. Finally, the thermodynamic quantities (Delta H degrees and Delta S degrees) associated with this protonation process were calculated and discussed.