Selective a-chlorination of propanoic acid to form 2-monochloropropanoic (MCA) and 2,2dichloropropanoic acid (DCA) was investigated in a laboratory-scale, semibatch reactor at 90±130 °C at atmospheric total pressure and in the presence of chlorosulfonic acid (ClSO 3 H) and 2,2dichloroethanoic acid (DCA') as catalytic agents and oxygen as a radical scavenger. The decomposition of the catalyst was investigated with sulfur analysis and UV-spectrometry. The studies revealed that the majority of sulfur remains in the reaction mixture, but is converted to an inactive form during the chlorination. The reasons may be the decomposition of ClSO 3 H and its reaction with propanoic acid. The kinetic experiments revealed autocatalytic and parallel formation of MCA and DCA, the selectivity being independent of Cl 2 concentration in the liquid phase. The experiments with DCA' also demonstrated that DCA' has a catalytic effect on the chlorination The experiments con®rmed the validity of a previously proposed reaction scheme for a-chlorination, which comprises the formation of the reaction intermediate (propanoyl chloride) from propanoic acid and ClSO 3 H, the acid-catalyzed enolization of the acid and a hydroxyl-chlorine exchange reaction. The acid-catalyzed enolization is the rate determining step in the reaction sequence. The kinetic data were ®tted to rate equations based on the reaction scheme.