Electrochemical Impedance Spectroscopy (EIS) is often used to characterize the quality of barrier coating systems, with the entire spectrum from 100 kHz to 10 mHz taking an hour or more to complete. During this time the coating is often changing if for no other reason than the diffusion of the electrolyte solution into the coating. When the saturated water volume fraction of the coating, the diffusion rate and the electrical response are all low, the EIS response is stable and Kramers-Kronig consistency checks are met. Higher rates of diffusion, water volume fraction, and electrical impedance response to the composite system can impact the resonance frequency and thus the relative impact on the real and imaginary components of the coating impedance. The impact on the impedance for several representative coating systems due to water uptake is simulated, along with the Kramers-Kronig consistency implications.