Recent theoretical and experimental research showed that the response of micro/nanocantilevers to detect materials is not always simply related to extra mass. Stiffness of adsorbates and surface stress-induced changes in the stiffness, arising from adsorption, can produce frequency shifts that are several times greater in magnitude than those induced by mass loading. Consequently, the calculated adsorbed mass does not fully represent the real adsorbed mass, making the measurements qualitative. Therefore, a proper method for measuring the stiffness of adsorbed layers has to be combined with resonance frequency measurement to quantitatively analyze changes in both the mass and the stiffness. This letter presents the theory for application of electrostatic pull-in instability for measuring the stiffness of adsorbates at the surface of cantilever resonators. (C) 2010 Elsevier B.V. All rights reserved.