Novel hydrophobic Bronsted acidic ionic liquids (HBAIL) were prepared and utilized as acid catalysts in organic reactions in water. HBAILs were demonstrated, for the first time, to be effective catalysts for Prins cyclization of styrene derivatives in water with a formaldehyde water solution. Many styrene derivatives could be successfully converted to the corresponding 1,3-dioxanes. Other dehydration reactions also proceeded well using HBAILs in water. After reactions, HBAILs could be easily recovered and reused without significant loss of activity.