Abstract: A reactor for chlorinating rubber has an inlet opening which closely surrounds the rubber material entering the reactor therethrough, and hence any tendency for escape of chlorine is countered by absorption of that chlorine by the entering completely non-chlorinated rubber. The chlorinated rubber emerging from the outlet opening of the chamber is immersed in water before leaving the reactor and hence the outlet is sealed, by the water, against escape of the chlorine treatment gas. The rubber and chlorine displacements through the reactor are counter current to one another.