[en] Bk(3) complexing in acetonitrile solutions is studied using the spectrophotometric method. The constants of BkH₂PO₄²⁺ and Bk(H₂PO₄)₂⁺ complex stability are determined. The stability constant of the phosphate complex of tetravalent berkelium Bk(H₂PO₄)₃⁺ is calculated by the shift value of the formal potential of the Bk⁴⁺/Bk³⁺ pair in these solutions (relatively to the standard one). It is shown that when holding acetonitrile phosphate solutions of Bk(4) the latter is slowly reduced up to Bk(3) by the reaction of the zero order. The rate constant of the reduction reaction decreases with growth of H₃PO₄ concentration