1. A series of 2-arylalkyl- and 2-(4′-alkyl)phenoxymethylbenzimidazoles was synthesized and evaluated as inhibitors of mixed-function oxidase activity in phenobarbitone- andβ-naphthoflavone-induced rat liver microsomes.2. Higher homologues of the 2-arylalkyl series were more potent inhibitors than lower homologues against all mono-oxygenase activities except anilinep-hydroxylation.3. Smaller 2-substituents were associated with relatively low-affinity reverse type I spectral binding behaviour, whereas larger substituents were associated with type I binding of high affinity.