The oxyanion derivatives of the chalcogens tellurium and selenium are toxic to living organisms even at very low levels. Bacteria have developed mechanisms to overcome their toxicity by methylating them. The structure of TehB from Escherichia coli has been determined in the presence of the cofactor analogues SAH (S-adenosylhomocysteine) and sinefungin (a non-hydrolysable form of S-adenosyl-L-methionine) at 1.48 angstrom (1 angstrom = 0.1 nm) and 1.9 angstrom respectively. Interestingly, our kinetic data show that TehB does not discriminate between selenium or tellurite oxyanions, making it a very powerful detoxifying protein. Analysis of the active site has identified three conserved residues that are capable of binding and orientating the metals for nucleophilic attack: His(176), Arg(177) and Arg(184). Mutagenesis studies revealed that the H176A and R184A mutants retained most of their activity, whereas the R177A mutant had 65 % of its activity abolished. Based on the structure and kinetic data we propose an S(N)2 nucleophilic attack reaction mechanism. These data provide the first molecular understanding of the detoxification of chalcogens by bacteria.