The oxidation reaction of solid hydrazodicarbonamide with chlorine in aqueous solution is experimentally studied in a 1 1 agitated glass vessel reactor with a four-blade paddle stirrer. Based on the film theory, a model is first developed for this complicated system by taking account of the gas-liquid and solid-liquid mass transfer, solid dissolution, and instantaneous reaction, especially the reduction of solid-liquid mass transfer interfacial area arising from the solid reactant dissolution. The model can reasonably and well describe the experimental data. According to the values of gas-liquid and solid-liquid mass transfer coefficients estimated with the proposed model, it is found that the gas-liquid mass transfer is a rate-controlling step in the majority of the reaction course, but the solid-liquid mass transfer becomes a rate-controlling step when the reaction is close to the final stage of the process.