A time-resolved electrochemical quartz crystal microbalance is described for the first time. In this equipment the frequency of the signal obtained equals the difference between the frequency of the working crystal resonator and the fixed frequency of a reference resonator. Taking this signal as the gate-controlling signal, a 16-bit synchronous counter counts the number of pulses from a stable high frequency time-base generator. The period of the signal with the differential frequency is measured as a function of time. After the measurement is complete, the data are transferred to the main memory of the host computer for further data processing. In this paper the mass change during the process of underpotential deposition of lead on a silver electrode is described with satisfactory accuracy and time resolution.