Abstract: Process is provided for isothermal chlorination of certain hydrocarbons wherein a gas containing the selected hydrocarbon is passed to a first elongated reaction zone, and a gas containing chlorine is passed into a second elongated gas zone positioned along said first zone, said first and second zones being separated by a porous member. The pressure of gases within the second zone is maintained in excess of the pressure in the first zone for diffusion of chlorine through the porous member into the first zone, which is maintained under conditions sufficient to effect reaction therein of at least a portion of diffused chlorine with the hydrocarbon for production of the desired chlorinated hydrocarbon. The reaction product containing the desired chlorinated hydrocarbon is discharged from the first zone.