The chlorination of acetic acid with molecular chlorine in the presence of chlorosulfonic acid and thionyl chloride as catalytic agents was studied in a laboratory-scale semibatch reactor operating at atmospheric pressure. Monochloroacetic acid was the main product, and dichloroacetic acid was formed in parallel as a byproduct. Chlorosulfonic acid was a more active catalytic agent than thionyl chloride owing to its acid catalytic effect in enolization of the real catalytic intermediate, acetyl chloride. When the whole amount of the catalytic agent was introduced in the beginning of the reaction decreasing chlorination rates were observed, whereas autocatalytic kinetics appeared when stepwise addition of the catalytic agents was applied. In the former case the decreasing reaction rates were explained by the decomposition of the catalytic agents following first-order kinetics. The autocatalytic effects were explained by a kinetic model involving acid-catalyzed enolization of acetyl chloride and chlorination of the enol as rate-determining steps. The kinetic model provided a good description of the experimental results.