This work proposes the use of a rigorous approach to the analysis of the fuel-cell diffusive resistances not only at the commercial scale, but also at the laboratory one. The attention used experimentally for checking diffusion resistances in electrodes, cells and stacks should imply the same attention in the corresponding data analysis techniques. For this reason, some corrections to the most common interpretation and correlation procedures have been introduced on the basis of the study of complex electrodes and comparing these ones with simple ones. In Part I of this work a theoretical discussion of some simple reference cases will be reported. In Part 2 some examples of applications to the interpretation of the experimental data will be given. (C) 2007 Elsevier B.V. All rights reserved.