High-pressure storage of hydrogen in tanks is a promising option to provide the necessary fuel for transportation purposes. The fill process of a high-pressure tank should be reasonably short but must be designed to avoid too high temperatures in the tank. The shorter the fill should be the higher the maximum temperature in the tank climbs. For safety reasons an upper temperature limit is included in the requirements for refillable hydrogen tanks (ISO 15869) which sets the limit for any fill optimization. It is crucial to understand the phenomena during a tank fill to stay within the safety margins. The paper describes the fast filling process of hydrogen tanks by simulations based on the Computational Fluid Dynamics (CFD) code CFX. The major result of the simulations is the local temperature distribution in the tank depending on the materials of liner and outer thermal insulation. Different material combinations (type III and IV) are investigated. Some measurements from literature are available and are used to validate the approach followed in CFX to simulate the fast filling of tanks. Validation has to be continued in the future to further improve the predictability of the calculations for arbitrary geometries and material combinations. (c) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.