The homogeneous oxidation of m-xylene (MX) to isophthalic acid (IPA) with acetic acid as solvent catalyzed by Co–Mn–Br catalyst was investigated at temperatures from 448.2 to 466.2 K by semicontinuous and continuous experiments. Based on the free radical chain reaction mechanism of hydrocarbon, one simplified kinetic model of MX oxidation with six parameters was developed involving MX, IPA, and important intermediates. The kinetic model fitted experimental data well, in which there was only one adjustable parameter, i.e., chain initiation rate constant corresponding to various temperatures, while other parameters were kept constant. It was found that the oxidation rate constants concerning the methyl on the benzene ring of MX was well consistent with the predictions by the Hammett structure–reactivity relationship. Through reasonable assumptions, the well-mixed reactor model of MX oxidation was further developed, by which successful predictions of the continuous MX oxidation process under different operating conditions were performed. Hopefully the kinetic experiments and related models may bring fundamental data and some valuable insights to industrial MX oxidation.