The relative surface area, R-sa, i.e., the ratio of the surface area in a three-dimensional space to that in a two-dimensional space: of quartz crystal vibrators, used for an electrochemical quartz crystal microbalance (EQCM), was determined by atomic force microscopy in air, by coulometry in solution, and derived from the Brunauer-Emmett-Teller isotherms for xenon adsorption. The resulting R-sa values were mutually consistent and were higher the better was the resolution of imaging.