A range of Lewis acidic metal chlorides were tested as homogeneous catalysts in a model Diels–Alder reaction; AlCl3 and GaCl3 performed best in terms of conversion and endo : exo selectivity. Based on this outcome, the corresponding chlorometallate(III) ionic liquids were tested as homogeneous catalysts, without a decrease in performance compared to the chlorides. Finally, these ionic liquids were heterogenised by covalent tethering of the cation onto a multimodal porous silica support; such supported chlorogallate(III) ionic liquids are reported for the first time. Supported chlorometallate(III) ionic liquids were used as recyclable heterogeneous catalysts for the Diels–Alder reaction under solventless conditions. High yields (99%) and high endo-selectivities (95%) were obtained after very short reaction times (5 min) at near-ambient temperature (25 °C).