In hydrogen fuelling stations hydrogen is usually stored in the high-pressure buffer or cascade storage systems. Buffer storage system includes a single pressure reservoir, while the cascade storage system is usually divided into three reservoirs at low, medium and high-pressure levels. In the present study, first and second laws of thermodynamics have been employed to analyze the filling process associated with these two storage systems. The important parameters such as filling time, filled mass and compressor input work have been examined in detail. Assuming the same final vehicle on-board in-cylinder pressure for both storage systems, the results reveal that filling time of the buffer storage system is much less than the cascade storage system. However, the filled mass related to the buffer system for the same conditions is approximately equal of the cascade system. Furthermore, the buffer system is accompanied with much higher entropy generation as compared to the cascade storage system, which directly reflects in the amount of required compressor input work. Entropy generation minimization has also been employed to determine the optimized low and medium-pressure reservoir pressures for the cascade storage system, which corresponds to the lowest required compressor input work for a specific high-pressure reservoir in the cascade systems. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.