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, wherein at least one metal salt is an aluminium salt and any further metal salt is a salt of a metal selected from the group consisting of Group IB elements of the Periodic Table, Group IIB elements of the Periodic Table and transition elements of the Periodic Table, 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 aluminium salt as anion which is the same as the aluminium salt present in the first ionic liquid. The process of the invention provides means to more efficiently run an ionic liquid alkylation process with less impact on the environment than processes known from the prior art.