Zeolite beta was dealuminated by thermal and acid treatment methods. Cyclisation of (phenylthio)acetaldehyde diethylacetal was carried out over these dealuminated zeolites with a view of obtaining benzo[b]thiophene. The yield of benzo[b]thiophene is higher in the presence of acid-dealuminated beta zeolites than in the presence of their thermally dealuminated counterparts. The product formation was thus greatly affected by the treatment methods. Thermally dealuminated beta zeolite unexpectedly favoured Meerwein-Ponndorf-Verley (MPV) reduction of the intermediate, namely, thiophenyl acetaldehyde rather than cyclisation. The different aluminium species present in the modified zeolites were analysed using DRIFT and 2 7 Al MAS-NMR spectra. Such unexpected MPV activity is correlated to the partly framework aluminium species.