Selectivities are presented of the Beckmann rearrangement of cyclohexanone oxime to -caprolactam with oleum for various conditions in three microreactors, viz., Y-junction, interdigital, and split and recombine mixers, followed by a 50-cm long microchannel of 250 m internal diameter. Cyclohexanone oxime is dissolved in cyclooctane, which is inert for oleum. The selectivity is measured in the temperature range of 80-132°C. The concentration range of caprolactam in the reaction mixture is 31-41 wt %, in oleum. The total volumetric flow rate is 0.4 ml/min, whereas the flow rate ratio of -caprolactam/oleum over cyclohexanone oxime/cyclooctane ranges from 0.3 to 3. The selectivities measured with the three microreactors are: 70-99+%, 93-99+%, and 95-99+%, respectively. High -caprolactam concentration (41 wt %), high temperature (110-132°C), and a ratio of free H2SO4 to SO3 of unity have a negative effect on the selectivity.