Abstract: Potassium antimonyltartrate (Ⅰ) is an effective schistosomacidal agent. It would be of interest to see the activity of its corresponding pentavalent antimony derivative. Furthermore, chelation of the latter compound with various hydroxy acids would give a series of new compounds with different degree of stability, which might have some relations with their pharmacological properties. Treatment of aqueous potassium antimonyltartrate (Ⅰ) with 30% hydrogen peroxide gave the corresponding pentavalent antimonyltartrate (Ⅱa). This compound is a weak acid, forms a di-potassium salt (Ⅱb) with potassium hydroxide but does not react with weak bases such as diethylamine or pyridine. Ⅱa, on boiling with potassium bitartrate, gave di-potassium antimonyl~(V) ditartrate (Ⅲ). This new compound could also be formed by the oxidation of antimony trichloride by means of hydrogen peroxide followed by chelation with tartaric acid and then neutralization with potassium hydroxide. Similar complexes were prepared by the action of glyceric acid, D-gluconic acid, mucic acid and gallic acid on Ⅱa. These new compounds are water-soluble substances, and they are suitable for injection. Preliminary pharmacological examination showed that they had quite low toxicity.