Acidic ionic liquids based on trihexyltetradecylphosphonium chloride-P6.6.6.14 Cl and six different metal chlorides (YCl3, YbCl3, MgCl2, ZnCl2, CuCl2, InCl3) were prepared. The molar fraction of metal chloride (XMClx) used for the formation of acidic ionic liquids with P6.6.6.14 Cl was changed in the range from 0.3 to 0.5. The high catalytic activity in the Diels-Alder reaction of cyclopentadiene and various αβ-unsaturated carbonyl compounds showed ionic liquids at XMClx≤0.4. In general, the highest product yields and