A general approach is proposed to investigate an electrochemical reaction mechanism by simultaneous electrochemical and electrogravimetric impedance measurements. Our approach shows that a.c. electrogravimetry may provide some useful information on the chemical nature of the species involved, in addition to the information obtained by using electrochemical impedance analysis alone. Application of a.c. electrogravimetry to passive iron in sulfuric acid solution showed that the major part of the charge transferred during a transient potential change, within the Fe passive potential range, contributes to iron corrosion rather than to the film growth. In the case of electropolymerization of polypyrrol in the presence of sodium dodecylsulfate, it was shown that the electropolymerization is followed by an oxidation process, which occurred in pores crossing the polymer.