This series of papers presents details of numerical studies of the nature of the impedance of solid oxide fuel cell (SOFC) anodes caused by gas-phase transport processes. The present part treats channel geometries where gases are transported parallel to the electrode surface. Two cases are investigated: (i) channel flow by forced convection, a typical situation in planar stack segments; and (ii) channel diffusion without convective flow, a typical situation in laboratory-scale single-chamber experiments using symmetrical cells. Current/voltage curves and electrochemical impedance spectra are simulated based on the Navier-Stokes transport equations and nonlinear electrochemistry models. Both channel flow and channel diffusion cause a capacitive behavior in the form of an resistance-capacitive (RC)-type semicircle in the Nyquist diagram. Its resistance and relaxation frequency strongly depend on operation parameters (gas concentration, flow rate, temperature, electrochemical polarization) and geometry (channel length and cross-sectional area). The model predictions are in good quantitative agreement with four different experimental studies published in the literature. The simulation approach thus allows a physically based assignment of observed gas concentration impedance processes. (c) 2007 The Electrochemical Society.