Electrochemical impedance measurements were carried out on symmetrical nickel/gadolinia-doped ceria test cells. For H-2, N-2, and H2O mixtures, the diffusion length obtained based on the impedance measurements is on the order of centimeters. This high value of the diffusion length is attributed to the flow field in the reactor. It is suggested that a detailed analysis of the gas flow field inside the test reactor is essential before interpreting the impedance measurements with various solid oxide fuel cell test configurations. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3231490] All rights reserved.