The Kramers-Kronig relations, which strictly apply for systems that are linear, stable, and causal, provide all essential tool for assessing the internal consistency of impedance data. The Kramers-Kronig relations tire understood to be sensitive to failures of causality, but insensitive to failures of linearity. Numerical simulations Were performed to explore the conditions under which the Kramers-Kronig relations are sensitive to nonlinear behavior of electrochemical systems. A characteristic frequency was identified below which the Kramers-Kronig relations are satisfied, even for very nonlinear systems. This result is observed for systems with a small ohmic resistance. The Kramers-Kronig relations are not satisfied for measurements which include the characteristic frequency. For systems with a large ohmic resistance, the Kramers-Kronig relations may provide a better tool for assessing the presence of nonlinear behavior as compared to analysis of low frequency Lissajous plots. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3190160] All rights reserved.