A process for producing a halogenated isoolefin copolymer involves contacting an unsaturated isoolefin copolymer cement, the cement containing an unsaturated isoolefin copolymer dissolved in an organic solvent, under halogenation conditions with a halogenating agent and an aqueous solution of an organic peracid oxidant to form a two-phase reaction medium having an organic phase and an aqueous phase, the organic peracid oxidant capable of converting hydrogen halide to free halogen. The process leads to higher halogen utilization even when the cement contains significant amounts of water and without the use of an emulsifier.