Fast ionic transport in microporous activated-carbon electrodes is a prerequisite for the effective energy storage in electrochemical supercapacitors(1,2). However, the quartzcrystal microbalance(3-8) (QCM), a direct tool to measure ionic fluxes in electrochemical systems, has not yet been used for studying transport phenomena in activated carbons (except for an early report on carbon nanotubes(9)). Conventional electroanalytical(1) and suitable surface and structure-analysis techniques(10-12) provide limited prognostic information on this matter. It has been demonstrated herein that the QCM response of typical microporous activated carbons can serve as a gravimetric probe of the concentration and compositional changes in their pore volume. This allowed direct monitoring of the ionic fluxes, which depended strongly on the electrode's point of zero change, pore width, ion size and cycling conditions ( polarization amplitude, charge/discharge depth and so on). The information on the nature of ionic fluxes into activated carbons is critical for promoting improvements in the performance of electrochemical supercapacitors, membrane technologies and (electro/bio) chemical sensors.