Cyclization of (phenylthio)acetaldehyde diethylacetal was examined in 1,2-dichloroethane and chlorobenzene solvents in the presence of H-beta zeolite and Zn2+ ion-exchanged beta zeolite under reflux conditions. Zn2+ ion-exchanged beta zeolite showed better activity than its H-form counterpart owing to its higher Lewis acidity. The cyclization proceeds via two different pathways. At lower temperature the reaction proceeds predominantly via a pathway in which deacetalization of the reactant occurs first and then the resulting aldehyde cyclizes to benzo[b]thiophene.