In this article, exergy losses of fluid motion in a microchannel are investigated. Thermal, friction and electromagnetic irreversibilities contribute to the total rate of exergy destruction. Additional input power from the externally applied electric field is needed to overcome these irreversibilities and deliver specified rates of mass and heat flow through the microchannel. Different cases of steady-state heat transfer in a non-polarized electromagnetic field are considered. Predicted results of fluid velocity, exergy destruction and optimal Reynolds number are presented and compared successfully against past data and measurements involving exergy destruction. It is shown that exergy destruction increases with stronger magnetic fields and wider microchannels. Furthermore, the optimal Reynolds number (minimizing the rate of exergy destruction) increases at lower microchannel aspect ratios. (c) 2005 Elsevier Ltd. All rights reserved.