The chlorination kinetics of propanoic acid was investigated in a semibatch reactor at 70-130 degrees C using chlorosulphonic acid as a catalytic agent and oxygen as a radical trapper. 2-Monochloropropanoic acid was obtained as the main product and 2,2- and 2,3-dichloropropanoic acids appeared as by-products. The formation kinetics of 2-monochloropropanoic and 2,2-dichloropropanoic acids were autocatalytic, whereas the kinetics of 2,3-dichloropropanoic acid was noncatalytic. The kinetic data were described with a rate model based on a reaction mechanism, where propanoyl chloride is the key intermediate. The rate determining steps in the mechanism are the acid catalyzed enolization of propanoyl chloride and the chlorination of the enol. The parameters of the rate model were estimated with nonlinear regression analysis. Simulations showed that the kinetic model is able to describe the autocatalytic formation kinetics of 2-monochloropropanoic and 2,2-dichloropropanoic acids.