A process for producing a chlorinated polyolefin is disclosed, which comprises dissolving or suspending a polyolefin in a solvent and conducting chlorination reaction of the dissolved or suspended polyolefin, said solvent being a chloroform-based solvent comprising a mixture of 100 parts by weight of chloroform containing no alcohol and from 0.0005 to 15 parts by weight of a compound having from 4 to 10 carbon atoms and containing one or more double bonds per molecule. A process for producing a chlorosulfonated polyolefin is also disclosed, which comprises dissolving or suspending a polyolefin in a solvent and conducting chlorination and chlorosulfonation reaction of the dissolved or suspended polyolefin, said solvent being a chloroform-based solvent comprising a mixture of 100 parts by weight of chloroform containing no alcohol and from 0.0005 to 15 parts by weight of a compound having from 4 to 10 carbon atoms and containing one or more double bonds per molecule.