The reactions of amidinothiourea NH2 -(S=) C-NH-C (=NH).NH2 with metal ions1 and their analytical applications2-5 are reported in the literature, but the protons-tion of amidinothiourea has not yet been investigated. The protonation reaction may be represented as follows: B + H+ \(\rightleftharpoons\) BH+ BH++ H+ \(\rightleftharpoons\) \(BH^{++}_2\) whero B represents a molecule of amidinothiourea. The first (K1) and the second (K2) formation constants of protonated amidinothiourea are given by \(K_1 = {[BH^+] \over [B][H^+]}\) and \(K_2 = {[BH_a^{4+}] \over [BH^+][H^+]}\) at constant ionic strength.