A novel process for the preparation of a chlorinated polyolefin having excellent low temperature characteristics and brightness is provided which comprises allowing a polyolefin dissolved or suspended in a solvent to undergo chlorination reaction with chlorine gas or chlorinated sulfuryl in the presence of a radical initiator as a catalyst, characterized in that as said solvent there is used 1,1,2-trichloroethane and the temperature is controlled to 90.degree. C. or lower at the process for removing hydrogen chloride and/or sulfur dioxide by-produced during the reaction from the reaction system. A further process for the preparation of a chlorosulfonated polyolefin having excellent low temperature characteristics and brightness is provided which comprises allowing a polyolefin dissolved or suspended in a solvent to undergo chlorosulfonation reaction with chlorine and sulfur dioxide, chlorine and sulfuryl chloride, sulfuryl chloride alone, chlorine, sulfur dioxide and sulfuryl chloride or sulfuryl chloride and sulfur dioxide in the presence of a radical initiator as a catalyst, characterized in that as said solvent there is used 1,1,2-trichloroethane and the temperature is controlled to 90.degree. C. or lower at the process for removing hydrogen chloride and/or sulfur dioxide by-produced during the reaction from the reaction system.