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Though electrochemical impedance spectroscopy (EIS) has been widely used in mechanistic investigations of electrocatalytic reactions, the correlation between the shape of an EIS diagram and the electrocatalytic activity is largely unclear; the complexity of electrocatalytic reactions in terms of reaction mechanisms and influencing factors casts an air of pessimism over the existence and, if it exists, the stability of such a correlation. Nevertheless, understanding the correlation can help select reaction mechanisms and electrical circuit models in data analysis, use the EIS shape as a descriptor of electrocatalytic activity, and detect side reactions if the measured shapes deviate from the theoretical prediction based on the main reaction. Herein, the problem is tackled by a systematic mathematical analysis of firstly single-adsorbate reactions and then more complicated reactions involving multiple adsorbates. A complete regime diagram of all possible EIS shapes of single-adsorbate reactions is provided, navigating the subsequent analysis in the multidimensional parametric space. For single-adsorbate reactions involving two steps with identical transfer coefficients of 0.5 in the absence of lateral adsorbate interactions, several rigorous remarks on the EIS shape are made. Unfortunately, many of them require modifications when different transfer coefficients and lateral interactions are considered. Nevertheless, several trends in EIS shapes are robust against complexities of reaction mechanisms and variations in reaction parameters, which also receive experimental evidence collected from the literature. The theoretical analysis presented here may trigger interest in finding a correlation between the impedance shape and the electrocatalytic activity across a wide range of reactions.