Liquid phase chlorination in an evaporatively cooled reactor of 1,3-butadiene to 3,4-dichlorobutene-1 and 1,4-dichlorobutene-2 by contacting 1,3-butadiene and elemental chlorine in a solvent at a temperature of 25.degree.-100.degree. C. said solvent being butane or pentane or a fluorinated solvent of the formula (CR.sub.3)(CR'.sub.2).sub.m (CR".sub.2).sub.n R where R is independently hydrogen, fluorine, chlorine or bromine, R' is hydrogen, R" is independently fluorine, chlorine or bromine, m and n are 0-3 with the proviso that terminal carbon atoms are independently perhalogenated or fully hydrogenated, the solvents boil at -15.degree. C. to 40.degree. C. at atmospheric pressure, and the solvent to dichlorobutene ratio is from 2.0:1 to 11:1. The heat of reaction is removed by vaporization of solvent and 1,3-butadiene and returning the ingredients to the reactor.