A two-fluid electroosmotic flow in a microchannel is studied by considering full hydrodynamic and electrostatic interactions on the interface. Jumps in electrical potential and in charge density across the interface, in particular, are found to create counterintuitive flow behavior through the electrostatic interaction of the interface with the external field imposed. The interfacial electrostatic effects are shown to induce flow reversal within physically reasonable parametric ranges. It is also shown that the electrostatic properties of the interface must be carefully considered in electroosmotic pumping lest the nonconducting fluid should stay stationary or flow in an unintended direction. A formula for quantitative control of electroosmotic pumping is provided. (C) 2011 Elsevier Inc. All rights reserved.