The ring-opening reactions of methyloxirane on various zeolites and zeotypes (HZSM-5, CuZSM-5, HY, AlMCM-41, SiMCM-41, and BMCM-41) were investigated at 363 K either in a pulse reactor or in a circulation system, in the presence of hydrogen or nitrogen. The acidic molecular sieves were found to be active in isomerization (the products are propionaldehyde and acetone) and dimerization (the products are dioxolane and dioxane derivatives) reactions. Deoxygenation was of minor importance in hydrogen or nitrogen atmosphere. The major product formation pathway was dimerization on every catalytically active material. This transformation route was favored by the confined environment.