The uncatalyzed selective oxidation of cyclohexane with air was performed at high-p, T-conditions in a microcapillary reactor. Operation pressures were between 2.0 and 8.0 MPa and operating temperatures between 453 and 533 K. The measured space-time-yield showed only a slight dependence on the pressure, but a strong dependence on temperature. At 533 K, a space-time-yield of about 6.000 kg/(m(3)h) was reached, which corresponds to a size of 2m x 2m x 2m (8 m(3)) of the microstructured reactor assuming a capacity of 100.000 t/a compared to 500 m(3) total reactor volume realized with a cascade of bubble columns of each about 100 m(3). Unfortunately, selectivity drops at this temperature below 80% which is significantly lower than the selectivity in the conventional process. Passivating the capillary walls with silicon allowed an increase in selectivity. By fitting a simple mass transfer/reaction model for molecular oxygen, the mass transfer coefficient could be determined for T 453 K and a 0.75 mm capillary as k(L) = 3.04 x 10(-3) m/s. With the help of the Hatta number, mass transfer limitations can be excluded for the microcapillary reactor, whereas the bubble column reactor is weakly limited by the gas/liquid mass transfer of the molecular oxygen. Thus, process intensification by enhancing mass transfer using a microstructured reactor for cyclohexane oxidation with air is quite low. (C) 2010 Elsevier Ltd. All rights reserved.