Twelve new dithiaarsanes were evaluated for their in vitro and in vivo trypanocidal properties in regard to their three parent molecules, 4-amino-phenylarsenoxide, melarsenoxide, and 4-dansylamino-phenylarsenoxide. The most potent dithiaarsane, compound 2b, had a minimum effective concentration of 1.5 nM after 48 h of incubation and at a dose of 0.39 µmol/kg of body weight (0.2 mg/kg) administered subcutaneously cured 100% of mice acutely infected with Trypanosoma brucei brucei CMP. With this model, the chemotherapeutic index of compound 2b was 512, compared to 256 for melarsamine dihydrochloride (Cymelarsan) under the same conditions. With a chronic infection produced by T. brucei brucei GVR, compound 2b cured 100% of mice after treatment at a dose of 25 µmol/kg (12.5 mg/kg) for 4 consecutive days, whereas melarsamine dihydrochloride and potassium melarsonyl (Trimelarsan) cured less than 50% mice at this dose. For both acute and late-stage infections, dithiaarsanes having a melaminophenyl ring exhibited the most-potent trypanocidal activity. Compound 2b is thus one of the most active organoarsenicals described in a mouse trypanosomiasis model. Considering that the main intracellular targets of organoarsenicals are thiol groups, we studied the possibility of ligand exchange between Cymelarsan and several dithiols. In aqueous solution, we observed a rapid exchange of cysteamine from melarsamine with free cysteamine and also with various dithiols always in favor of more stable cyclic derivatives. These ligand exchanges suggest the ability of trivalent organoarsenicals to react with targets such as trypanothione and dihydrolipoic acid. Among several ligands, a 1,3-dimercaptopropane moiety appeared the most suitable for trypanocidal activity.