Many electrochemical processes contain more than one charge transfer step. If one of these steps leads to a mass change of the working electrode one can use the electrochemical quartz crystal microbalance (EQCM) in situ to determine the rate of this process as a function of the applied potential. Examples for the use of the EQCM for the elucidation of complex electrochemical reaction, which are discussed in this paper are the electrodeposition of alloys with simultaneous hydrogen evolution, the corrosion of alloys, and the autocatalytic reduction of hydrogen peroxide on silver electrodes in acidic solution.