Tungstophosphoric acid (Hg PW12O40) supported on silica gel proved to be an active heterogeneous catalyst for Diels-Alder reactions of enones with various dienes. The Diels-Alder adducts were formed in high regio- and/or stereoselectivity. In some cases the Diels-Alder reactions were accompanied by competing polymerization of the starting compounds. We also found that trifluoromethanesulfonic acid (triflic acid) is an effective homogeneous catalyst for the reaction of unprotected but-3-en-2-one (methyl vinyl ketone) with 1,3-cyclohexadiene, albeit with a lower activity than tungstophosphoric acid.