Impedance spectroscopy is a very useful technique as a testing method in analysis of electrochemical systems, material studies, corrosion systems, batteries, etc. However its use in fuel cells has been relatively limited. This work presents impedance results during MEAs evaluation, when testing conditions are varied in single cells. Contributions of different processes effects on complex plane results are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.