Highly exothermic reactions such as aromatic nitration can be carried out in a highly efficient manner in microreactors due to the high heat and mass transfer rates in such reactors. In this study, nitration of toluene was conducted in a microreactor immersed in a constant temperature bath using concentrated nitric acid as the nitrating agent. The effects of residence time, reaction temperature, and acid concentration, etc., on the reactor performance were studied. The results indicate that in the microreactor the nitration reaction proceeds rapidly, even without sulfuric acid, probably by self-protonation of nitric acid. Also, no by-products were detected. The isomeric distribution of the product nitrotoluenes was found to be identical to that generally found in the mixed-acid nitration. The results were compared with the data obtained from a batch reactor, and the microreactor reaction rates were found to be much higher than the batch reactor rates. (c) 2007 Elsevier B.V. All rights reserved.