The present invention relates to a regeneration process for producing a regenerated ionic liquid catalyst from solids formed in an ionic liquid alkylation process wherein a first ionic liquid is used as a catalyst which is a composite ionic liquid comprising ammonium cations, and anions being composite coordinate anions derived from two or more metal salts, the regeneration process comprising (a) removing the solids from the reaction zone of the alkylation process; and (b) subsequently treating the solids with a second ionic liquid made from an ammonium salt as cation, and an aluminum salt as anion which is the same as the aluminum salt present in the first ionic liquid.