Propanoic acid was selectively chlorinated to 2-chloropropanoic acid in a laboratory scale semibatch reactor operating at atmospheric pressure and in a temperature range of 70-110 degrees C. Chlorosulfonic acid was used as a catalytic agent and molecular oxygen as a radical scavenger. 2,2- and 2,3-dichloropropanoic acids were formed as byproducts. The chlorination kinetics was autocatalytic, when the catalyst concentration was maintained constant by addition. When all of the catalyst was introduced in the beginning of the experiment, a decreasing chlorination rate was observed because of decomposition of the catalyst. The primary reaction product from catalyst decomposition was sulfopropanoic acid, which under the reaction conditions is probably decomposed to gaseous products. The explanation of the autocatalysis is the increase of the acidity of the liquid phase during the progress of the reaction. The autocatalytic kinetics was explained with a reaction mechanism involving the acid-catalyzed enolization of the key intermediate, propanoyl chloride, as a rate-determining step. In the absence of air a radical mechanism contributed to the overall kinetics, which resulted in polychlorinated products. Rate equations based on the autocatalytic ionic mechanism and radical chlorination mechanism were derived, and the kinetic parameters included in the equations were estimated with nonlinear regression analysis from the semibatch reactor data.