In the title compound, [Co(SO(4))(C(12)H(8)N(2))(2)]·C(4)H(10)O(2), the Co(2+) ion has a distorted octa-hedral coordination environment composed of four N atoms from two chelating 1,10-phenanthroline ligands and two O atoms from an O,O'-bidentate sulfate anion. The dihedral angle between the two chelating N(2)C(2) groups is 83.48 (1)°. The Co(2+) ion, the S atom and the mid-point of the central C-C bond of the butane-2,3-diol solvent mol-ecule are situated on twofold rotation axes. The mol-ecules of the complex and the solvent mol-ecules are held together by pairs of symmetry-related O-H⋯O hydrogen bonds with the uncoordinated O atoms of the sulfate ions as acceptors. The solvent mol-ecule is disordered over two sets of sites with site occupancies of 0.40 and 0.60.