The role of acetyl chloride as the active catalyst in the chlorination of acetic acid to mono- and dichloroacetic acids was examined by chlorinating acetic acid and monochloroacetic acid in the presence of acetyl chloride in semibatch and batch reactors. The reaction kinetics was autocatalytic when the acetyl chloride concentration was maintained constant by addition. If the acetyl chloride was added only at the beginning of the experiment the reaction rate decreased owing to the volatilization of acetyl chloride. The volatilization increased at high acetic acid conversions, since the solubility of acetyl chloride is lower in monochloroacetic acid than in acetic acid. The chlorination kinetics was described by a reaction mechanism consisting of acid-catalysed acetyl chloride enolization followed by enol chlorination and chlorine exchange steps. According to the mechanism the enol chlorination proceeds in parallel to the formation of mono- and dichloroacetyl chlorides. This was verified in separate experiments, in which acetyl chloride was chlorinated in the presence and in the absence of an acidic enolizing agent, chlorosulphonic acid. The chlorine exchange reactions were verified by studying the acetyl chloride-monochloroacetic acid and the acetyl chloride-dichloroacetic acid equilibria. Chlorination of monochloroacetic acid in the presence of acetyl chloride revealed that dichloroacetic acid is formed mainly via the chlorination of acetyl chloride to dichloroacetyl chloride, which undergoes chlorine exchange with monochloroacetic acid.