Abstract: The chromium(Ⅲ) ion is of characteristic by hydrolysis reaction. Its second hydrolysis-product Cr(OH)₂⁺ always coexists together with polynucl-ear species, but this complicated case was not takeninto account while determinating hydrolysis constant of Cr(OH)₂⁺. Therefore the result obtained by previous authors is unsatisfactory. We proved first by calorimetric titration that the first hydrolysis product CrOH²⁺ can exist alone, but Cr(OH)₂⁺ can not within certain range of pHvalue and furthermore obtained the first and second hydrolysis constants by curve-fitting under considering the coexistence of mono- and polynuclear species. The result is as follows: lgβ₁₁ = -4.15±0.04, lgβ₂₁ = -8.40 ±0.03. The comparision with the previous results has deen made.