An insulating layer can develop on the electrode surface during the analysis or disposal of many phenolic compounds due to electro-polymerization. This could cause electrode passivation, also called electrode fouling, which is indicated by a continuously decaying current signal. This paper attributed the cause of passivation to the potential drop across the fouling layer. The potential drop was observed and quantitatively measured by using the method of copper deposition. A scientific model based on the potential drop was proposed and fitted to the experimental results; this model is helpful for deep understanding and predicting the electrode passivation/fouling process. (C) 2014 The Electrochemical Society. All rights reserved.