This paper describes a dynamic fuel cell vehicle simulation (FCVSim) tool for the load-following direct-hydrogen (DH) fuel cell vehicle. The emphasis is on simulation of the direct-hydrogen fuel cell system (FC System) within the vehicle simulation tool. This paper is focused on the subsystems that are specific to the load-following direct-hydrogen model. The four major subsystems discussed are the fuel cell stack, the air supply, the water and thermal management (WTM), and the hydrogen supply. The discussion provides the details of these subsystem simulations. The basic vehicle configuration has been previously outlined by Hauer [An Analysis Tool For Fuel Cell Vehicle Hardware and Software (Controls) with an Application to Fuel Economy Comparisons of Alternative System Designs, Dissertation, UC California, Davis, USA, 2001] and Hauer and Moore [Fuel Cells for Automotive Applications, Professional Engineering Publishing, 2003, pp. 157-177, ISBN 1860584233] and is only briefly reviewed in this paper. (c) 2004 Elsevier B.V. All rights reserved.