In this paper, the micromixing efficiency in a membrane reactor was studied by employing competing parallel reactions. In the membrane reactor, one liquid is added gradually into another one through the nano-scaled micropores of ultrafiltration membranes. The effects of molecular weight cut-off (MWCO) of membranes, permeation fluxes, Reynolds number and reactants concentrations on the segregation index:were investigated. The results show that the decreasing membranes MWCO and the increasing Reynolds number favor the enhancement of micromixing efficiency. Compared with the other reactors (e.g. stirring tank, ultrasound, static mixer, Coutte flow reactor), the membrane reactor exhibits more excellent effects in intensifying micromixing. (c) 2005 Elsevier B.V. All rights reserved.