The comparison of the mathematical model and experimental results of the free radically initiated semi-continuous emulsion polymerization of styrene/butadiene/acrylic acid in the presence of chain transfer agent (CTA) is presented. The mathematical model takes into account the mechanism of gel formation and the effect of acidic monomers. It has also been included the inhibition caused by oxygen and monomer inhibitor. The experimental results (solids content, unreacted amount of monomer and gel content) and those obtained by the model are in rather good agreement. (C) 2003 Elsevier B.V. All rights reserved.