An appropriate oxygen excess ratio could improve the net power of a fuel cell system and reduce the potential of oxygen starvation. Due to nonlinear and time-varying characteristics, it is a difficult task to maintain the oxygen excess ratio within an ideal range particularly during transients. An adaptive control algorithm is therefore proposed to dynamically stabilize the oxygen excess ratio around an optimal level. The application of the algorithm inevitably involves the estimation of time-varying parameters and the pole assignment of the closed-loop system. The proposed adaptive control is evaluated on a test bench, and its application to overcome the time-varying problem is demonstrated. (c) 2008 Elsevier B.V. All rights reserved.