The flow in the Kenics static mixer is investigated in detail both numerically and experimentally in the range of Re = 100(...)1000. It was found that at Re = 300 the flow becomes unsteady. Two numerical methods, the lattice Boltzmann (LB) method and FLUENT, were compared and used to simulate the flow. The LB method proved to be a relatively fast and cheap (in terms of memory) alternative for the simulation of the transient flow in the Kenics static mixer at Re > 300. Furthermore, the flow field and dynamic behavior were validated by means of LDA experiments. The transient behavior observed could be explained by studying the dynamics of the vortices in the flow. The transition to unsteady flow takes place, when the vortices start to stretch out over an entire mixing element and create a disturbance in the flow, which subsequently triggers the unsteady behavior. (C) 2004 American Institute of Chemical Engineers.