A thermodynamic analysis is carried out for electrokinetic energy conversion. We demonstrate that the efficiencies depend solely on the figure of merit Z and are independent of the working mode (generator or pump) at the conditions of maximum power output and maximum efficiency. Between these two extreme points, the ratio of output powers and the ratio of operating conditions (generation voltage or pumping pressure) are also functions of only Z and independent of the working mode. The figure of merit Z associated with phenomenological coefficients is less than one due to the intrinsic entropy production in electrokinetic flows. We establish the phenomenological coefficients through electro-hydrodynamics, and find that Z is dependent on the non-dimensional electrokinetic radius, normalized zeta potential and non-dimensional parameter beta characterizing the liquid property. The narrow range of realistic beta limits the magnitude of Z to between 0.168 and 0.465 for aqueous solutions. The corresponding maximum efficiency thus varies between 4.59 and 15.51%. Within this range the electrokinetic devices' performance at maximum efficiency is, however, close to that at maximum power output. This feature is identified in a characteristic performance curve of efficiency against output power. (c) 2005 Elsevier B.V. All rights reserved.