A quartz crystal microbalance (QCM) having the capability of detecting the mass change of nanogram range was used to examine various interfacial processes at self-assembled monolayers (SAMs) of thiols, such as formation and desorption of SAMs, redox of ferrocene-terminated SAMs and acid dissociation of SAMs bearing -COOH and -NH₂ functionalities. From the mass change due to ion association between charged functionalities of SAMs and electrolyte ions, the change in the microenvironment around the functionalities during the redox of SAM, and acid-base properties of SAMs both on open circuit and under potential control were evaluated. Because of high sensitivity of QCM and unique information obtained by using it, QCM will contribute greatly to the progress of nanoscience and nanotechnology, particularly those related to solid surfaces.