The reductive removal of aromatic hydroxyl functions plays an important role in the anaerobic metabolism of many phenolic compounds. We describe a new enzyme from a denitrifying Pseudomonas sp., 4-hydroxybenzoyl-CoA reductase (dehydroxylating), which reductively dehydroxylates 4-hydroxybenzoyl-CoA to benzoyl-CoA. The enzyme plays a role in the anaerobic degradation of phenol, 4-hydroxybenzoate, p-cresol, 4-hydroxyphenylacetate, and other aromatic compounds of which 4-hydroxybenzoyl-CoA is an intermediate. The enzyme is therefore induced only under anoxic conditions with these aromatic substrates, but not with benzoate or under aerobic conditions. A similar enzyme which reductively dehydroxylates 3-hydroxybenzoyl-CoA is induced during anaerobic growth with 3-hydroxybenzoate. The soluble enzyme 4-hydroxybenzoyl-CoA reductase was purified. It has a molecular mass of 260 kDa and consists of three subunits of 75, 35, and 17 kDa. The subunit composition is likely to be a2b2c2. The enzyme contains 12 mol iron/mol and 12 mol acid-labile sulfur/mol and exhibits a typical ultraviolet/visible spectrum of an iron-sulfur protein. The reaction requires a reduced electron donor such as reduced viologen dyes; no other co-catalysts are required, the product is benzoyl-CoA and oxidized dye. The reductase is rapidly inactivated by oxygen. The inactivation by low concentrations of cyanide or azide in a pseudo-first-order time course suggests that it may contain a transition metal in an oxidation state which reacts with these ligands. 4-Hydroxybenzoyl-CoA reductase represents a type of enzyme which is common in anaerobic aromatic metabolism of phenolic compounds. A similar enzyme is demonstrated in Rhodopseudomonas palustris anaerobically grown with 4-hydroxybenzoate. The biological significance of reductive dehydroxylation of aromatics and a possible reaction mechanism similar to the Birch reduction are discussed.