Alkenes smoothly undergo condensation with paraformaldehyde in the presence of 10 mol% indium tribromide in ionic liquids (ILs) such as 1-butyl-3-methylimidazolium hexafluorophosphate [bmim]PF₆ or 1-butyl-3-methylimidazolium tetrafluoroborate [bmim]BF₄ under mild reaction conditions to afford the corresponding 1,3-dioxane derivatives in excellent yields with high selectivity. The recovered ionic liquid containing catalyst has been recycled in three to four subsequent runs.