The ether-cleaving O-demethylase isolated from syringate-grown cells of Acetobacterium dehalogenans (formerly named strain MC) consists of four proteins, components A, B, C and D. The enzyme system converts only phenyl methyl ethers with a hydroxyl group in the ortho position to the methoxyl moiety. The presence of a carboxyl group in the aromatic compound was not required for O-demethylase reaction. Component B mediated the conversion of vanillate to 3,4-dihydroxybenzoate in the presence of the Ti(III)-reduced corrinoid-containing component A. After addition of component D and tetrahydrofolate, methyl tetrahydrofolate was formed from vanillate in stoichiometric amounts. Titanium(III) citrate as a reductant could be replaced by H2, methyl viologen or ferredoxin, partially purified hydrogenase, purified component C obtained from A. dehalogenans, and ATP. From these findings, it was deduced that component B serves as vanillate:corrinoid protein methyltransferase (methyltransferase I) mediating the methyl transfer from vanillate to the reduced corrinoid protein component A. Component D functions as methylcorrinoid protein:tetrahydrofolate transferase (methyltransferase II). The role of component C is probably that of an activating protein reversing accidental oxidation of the protein-bound cob(I)alamin to cob(II)alamin in the presence of ATP and reducing equivalents supplied by the enzymatic oxidation of hydrogen.