Sulfite-oxidizing enzymes are essential for many living cells. Sulfite occurs naturally in the environment, and through metabolic pathways such as the degradation of sulfur-containing amino acids, organosulfonate metabolism and sulfur oxidation pathways in chemolithoautotrophic bacteria. Due to its highly reactive nature, the sulfite anion can react with vital cell components such as DNA and proteins. Therefore, cells need to be able to detoxify sulfite efficiently by oxidation to sulfate, facilitated by the sulfite oxidizing enzymes (SOE’s).  Here, we present the structure of the sulfite dehydrogenase enzyme (SorT) from Sinorhizobium meliloti both in isolation and in complex with its cognate electron acceptor SorU (a cytochrome c).  The three-dimensional structure of the complex defines a pathway for electron transfer between the SorT active site and the heme c cofactor of SorU, with conformational changes in the structure of SorU acting to facilitate inter-protein electron transfer. Structural comparisons between the ‘transient’ SorT/SorU protein-protein complex and related ‘permanent’ heterodimeric complexes, reveal a number of structural features, which are unique to the electron-transfer complex.  These presumably play a role in the fine-tuning of the complex as a ‘compromise’ between the specificity and the fast association/dissociation required for rapid electron transfer.