The title compound, 2-B(10)H(9)NH=C(CH(3))N(CH(3))CH(2)CH(2)CH(2)N(CH(3))(2)H·2C(2)H(6)OS or C(8)H(29)B(10)N(3)·2C(2)H(6)OS, is zwitterionic with the negative charge localized on the deca-borate cage and the positive charge on the terminal ammonium group. Two mol-ecules of dimethyl sulfoxide (DMSO) and one mol-ecule of the title compound constitute the asymmetric unit. One DMSO mol-ecule is disordered [ratio 0.739 (3):0.261 (3)]. The bonds and angles within the deca-borate cage are within the normal ranges. The amidine fragment of the ligand, which is expected to be planar, is significantly distorted from planarity as exemplified by four torsion angles [B-N-C-C = 8.4 (3), H-N-C-N = 5(2), N-C-N-C = 7.3 (3) and C-C-N-C = 14.8 (3)°] found within this portion of the mol-ecule. The crystal packing consists of head-to-tail-arranged dimers of the title mol-ecule held together by four mol-ecules of DMSO which are attached via strong N-H⋯O and weak C-H⋯O hydrogen bonds.