It has been known for some time that fuel cells operate in multiple time scales due to phenomena of different nature that govern their dynamics (electrical, chemical, mechanical, electrochemical, thermodynamic). It is expected that multi-time scales will be presented in the hydrogen gas reformer (produces hydrogen from hydrogen rich fuels) due to its complex chemical and physical nature. In this paper, we show how to present a linearized hydrogen gas reformer dynamic model in the coordinates in which slow and fast variables are explicitly defined and exactly separated. This result is important in its own right to get a better understanding of slow and fast dynamics of particular variables, study their interactions, and as well for designing reduced-order controllers and estimators for the hydrogen gas reformer that will independently operate in different time scales. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.