This paper reports a combined experimental and modeling study to explain the influences of detailed surface chemistry within solid-oxide fuel cell (SOFC) composite anode structures on electrochemical impedance spectra (EIS). Typically, SOFC models either neglect surface chemistry or include global reforming and water-gas-shift reactions using empirical rate expressions. In addition to improving the fidelity of representing heterogeneous chemistry within the anode, incorporating detailed surface chemistry is found to introduce capacity effects that alter the EIS. The model predicts frequency shifts that are measured in the complex impedance of SOFCs operating on hydrogen, carbon monoxide, and syngas mixtures. The significant influence of detailed heterogeneous chemistry on SOFC EIS is reported for the first time. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.046207jes] All rights reserved.