The Nuclear Hydrogen Initiative (NHI) of the U.S. Department of Energy's Office of Nuclear Energy Science and Technology is supporting an effort to reevaluate thermochemical cycles reported in the literature as having both promising efficiencies and proof-of-concept results. Recognizing that the calculation of efficiencies was not always consistently done or well defined in the literature, we first developed a consistent methodology for reevaluating the candidate thermochemical cycles. This methodology was defined for three levels of maturity in process knowledge. Argonne National Laboratory and a group of universities recalculated the efficiency for each level and identified the most critical R&D necessary to further assess the cycles' potential. This methodology is illustrated with the Cu-Cl in Part I of this series of three papers. Cur-rent results of the analyses for all of the cycles are summarized in Part II. Part III contains a more detailed Level 3 analysis for the Cu-Cl cycle. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.