Selective hydroxylation of phenol to benzenediols with hydrogen peroxide aqueous solution over titanium silicalite-1 (TS-1) catalyst was performed using microstructured catalytic wall reactors. We investigated effects on the reaction results of various reaction conditions, such as reactant conversion and product selectivities were investigated. The regioselectivity of the parallel reactions, i.e., the molar ratio of Para-diol (hydroquinone) to ortho-diol (catechol), is known to be dependent on the reaction parameters, such as temperature and reactant concentrations. In this study, we found that the selectivity also depends on the characteristics of the microreactor, such as catalyst placement and sizes of microchannels, which contributed significantly to the reactant concentrations on the catalyst surface. Especially, the selectivities changed markedly with increases in the height of microchannels. The results indicated that the regioselectivities could be arranged linearly as a function of Sh number, expressed by the velocity u and the design parameters of the microreactor, i.e., microchannel height H and microchannel length L. (c) 2007 Elsevier B.V. All rights reserved.