Three and two protonation macroconstants as well as twelve and four microconstants have been determined for arginine and citrulline, respectively. These data include microscopic basicity values of the arginine guanidino group, for which no reliable constants have been reported earlier. The determinations were carried out by a combination of potentiometric and deductive methods. The guanidino basicity proved to be extremely high, and can be characterized by microconstants between 14.7 and 15.0 log k units. The interactivity parameters could also be determined from the microscopic protonation constants. These values for the guanidino-amino and the guanidino-carboxylate interactions are 0.2 and 0.1 log k units, respectively. The pH-dependent distribution of all eight arginine microspecies is made visual by a microspeciation diagram. The microspeciation diagram is the most useful tool to predict bidentate binding isomerism in metal complex formation and site-specific bioligand-bioligand associations.