The mechanism of Beckmann rearrangement of cyclohexanone oxime in oleum is investigated with a multiphase microchemical system, which is designed to give good control of the reaction temperature and residence time. The influences of SO3 and caprolactam concentrations in oleum on the conversion are investigated. The results indicate that SO3 acts as the catalyst and caprolactam plays a product inhibition role. Based on these results and previous mechanism analysis, an equilibrium relation between protonated caprolactam and caprolactam hydrogen sulfate is proposed and the equilibrium constants at 70, 80, and 90 degrees C are gained. According to the equilibrium, the lowest acid/oxime molar-ratio of 0.5 for sufficient conversion (>99%) at 100 degrees C has been provided. This quantitative mechanism analysis gives the reason for the negative influences of higher caprolactam concentration and lower temperature on the reaction conversion, which is very useful for the optimization and reliable design of Beckmann rearrangement processes. (c) 2011 American Institute of Chemical Engineers AIChE J, 58: 31563160, 2012