The oxidation of the pseudohalide thiocyanate (SCN(-)) by Euphorbia peroxidase, in the presence or absence of added calcium, is investigated. After incubation of the native enzyme with hydrogen peroxide, the formation of Compound I occurs and serves to catalyze the thiocyanate oxidation pathways. The addition of a stoichiometric amount of SCN(-) to Compound I leads to the native enzyme spectrum; this process clearly occurs via two electron transfers from pseudohalide to Compound I. In the presence of 10 mM calcium ions, the addition of a stoichiometric amount of SCN(-) to Compound I leads to the formation of Compound II that returns to the native enzyme after addition of a successive stoichiometric amount of SCN(-), indicating that the oxidation occurs via two consecutive one-electron transfer steps. Moreover, different reaction products can be detected when the enzyme-hydrogen peroxide-thiocyanate reaction is performed in the absence or presence of 10 mM Ca(2+) ions. The formation of hypothiocyanous acid is easy demonstrated in the absence of added calcium, whereas in the presence of this ion, CN(-) is formed as a reaction product that leads to the formation of an inactive species identified as the peroxidase-CN(-) complex. Thus, although monomeric, Euphorbia peroxidase is an allosteric enzyme, finely tuned by Ca(2+) ions. These ions either can enhance the catalytic efficiency of the enzyme toward some substrates or can regulate the ability of the enzyme to exploit different metabolic pathways toward the same substrate.