This Letter studies theoretically the effects of electrochemical reactions having short time constants on the electrochemical impedance spectroscopy (EIS). We have shown that fast reactions are not detected by EIS no matter how high observation frequencies we use when the relaxation of electrode potential is slow, or when the electrochemical contribution of fast reactions is weak at the stationary state where the impedance is measured. Then only the kinetic parameters of slow reactions are measurable, and they are the effective ones modified by fast reactions. The form of the modification and the analytical representation of the impedance are given explicitly. (C) 2011 Elsevier B. V. All rights reserved.