An experimental study of the bulk-free radical copolymerization of styrene (STY)/ethyl acrylate (EA) initiated by 2,2’-azobisisobutyronitrile was conducted. Reactivity ratios were evaluated using both nonlinear least-squares (NLLS) and error-in-variables model (EVM) techniques. A thorough study of the kinetics over the full conversion range was subsequently carried out at a variety of feed compositions, initiator concentrations, and temperatures, with and without added chain transfer agent (CTA).