This paper adopts a sandwich-beam model with two surface layers of finite thickness to investigate the resonance frequency shift of moderately thick microbeams due to atom/molecule adsorption in presence of surface effects. Timoshenko beam theory is employed to take into account the effects of shear deformation and rotary inertia. It is found that, in addition to surface effects, the resonance frequency could perceptively depend on adsorption-induced mass and bending stiffness variation of the micro-beam which in turn depend on the adatom category and the substrate material. Adsorption and surface effects on the first three modes of vibrations are also studied, and comparisons are made with the results obtained on the basis of Euler-Bernoulli beam theory. This study is of interest for the optimal design of micro- and nanobeam-based sensors. (C) 2011 Elsevier B.V. All rights reserved.