A process for producing a crosslinked polymer involves contacting an epoxidized unsaturated isoolefin copolymer with a catalytic amount of an acid in an absence of a solvent. A process for producing a hydroxylated unsaturated isoolefin copolymer involves contacting an epoxidized unsaturated isoolefin copolymer with a catalytic amount of an acid in an absence of a solvent. A hydroxylated unsaturated isoolefin copolymer having hydroxyl groups in endo configurations may be produced thereby.