The crystal structure of the title compound, [Co(C2N2H8)2(C3N2H4)(SO3)] ClO4, 2H2O has been determined by X-ray diffraction, using diffractometer data, at 295 K, and refined by full-matrix least squares to a residual of 0.071 for 3425 'observed' reflections. Crystals are orthorhombic, Pcmn, a 15·822(9), b 11·924(6), c 9·261 (2) Å, Z 4. The imidazole and sulfite ligands are trans in the cation, which has m symmetry (Co-S 2·223(2); Co-N 2·008(4) Å). <Co-N(en)> is 1·964Å. The apparent thermal motion of the sulfite about the Co-S axis is very high. The internal internuclear distances are compared to the data reported for series of similar complexes in which trans effects are observed.