The selectivity and conversion of the sulfuric acid-catalyzed Beckmann rearrangement of cyclohexanone oxime, dissolved in cyclooctane, to ?-caprolactam are determined in a microreactor for conditions with a high concentration of ?-caprolactam. The microreactor consists of a low-temperature mixing zone followed by a high-temperature reaction zone. The mixing is conducted in a split-and-recombine micromixer and a microchannel at 65 degrees C, followed immediately by a second microchannel at 100-127 degrees C to obtain complete conversion. Under these conditions a selectivity of 99?% is achieved. The residence time of the reactants in the microreactor setup is about 10?s. In literature, a selectivity of about 95?% for the same reaction in a similar setup is reported, but at a uniform temperature of 120-130 degrees C for mixing and reaction. So, suppressing the reaction during mixing is a major tool to enhance the selectivity to ?-caprolactam.