We report the control of flow direction of a dilute salt solution in a microchannel by external voltage control of an ac electrokinetic micropump. Reversal of the flow velocity is achieved when a voltage of 2.0 V is applied to the drive electrodes and we show that the reverse velocity increases non-linearly with voltage above this value. Velocities of more than 250 um/s at a distance of 75 μm above the electrode surfaces have been measured, which is much greater than in the forward direction. A possible mechanism based on faradaic charge injection is proposed to explain the onset and propagation of reverse flow. We have also shown how multi-directional flow control can be utilized to promote mixing of fluids in a microchannel, and suggest a biochip application which can benefit from this new pumping technology.