In this paper, voltage sensors were developed to explore the voltage distribution characteristics inside the fuel cell under both steady and transient states. The effects of air stoichiometry and current density on the voltage distribution. under steady state were discussed, and the dynamic voltage response due to the load change under transient state was also investigated. Results showed that under transient state, the fuel cell would experience a temporary voltage fluctuation due to the air starvation. Thus could probably lead to the degradation of materials, such as the catalyst, membrane, etc. To lessen the degree of air starvation, a method of pre-supplying certain amount of air before loading was adopted. The relationship between the voltage response at the loading transient and the amount of pre-supplied air was also studied, and a minimum value of the pre-supplied air was obtained. The experimental results of this paper could be applied to the optimization of vehicular fuel cell system. (C) 2007 Elsevier B.V. All rights reserved.