A novel cytochrome ba complex was isolated from aerobically grown cells of the thermoacidophilic archaeon Acidianus ambivalens. The complex was purified with two subunits, which are encoded by the cbsA and soxN genes. These genes are part of the pentacistronic cbsAB-soxLN-odsN locus. The spectroscopic characterization revealed the presence of three low-spin hemes, two of the b and one of the alpha-type with reduction potentials of +200, +400 and + 160 mV, respectively. The SoxN protein is proposed to harbor the heme b of lower reduction potential and the heme alpha, and CbsA the other heme b. The soxL gene encodes a Rieske protein, which was expressed in F. coli: its reduction potential was determined to be +320 mV. Topology predictions CbsB and CbsA should contain 12, 9 and one transmembrane alpha-helices, respectively, with showed that SoxN, SoxN having a predicted fold very similar to those of the cytochromes b in bc(1) complexes. The presence of two quinol binding motifs was also predicted in SoxN. Based on these findings, we propose that the A. ambivalens cytochrome ba complex is analogous to the bc(1) complexes of bacteria and mitochondria, however with distinct Subunits and heme types.