The title compound, C(36)H(44)B(2)O·0.5CH(2)Cl(2), contains an almost linear O-B-O linkage [177.23 (15)°] and approximately orthogonal [interplanar angles 89.49 (5) and 80.77 (4)°] trigonal planar B centers, consistent with the previously reported non-solvated structure [Cardinet al. (1983). J. Chem. Res. (S), p. 93]. Inter-molecular C-H⋯π inter-actions exist between mesityl groups, with a C-H⋯centroid separation of 3.6535 (18) Å. The dichloromethane mol-ecules lie on twofold rotation axes.