Resonant mode micromechanical devices have great potential due to their high sensitivity and relatively easy signal processing. As they are also sensitive to environmental effects, vacuum packaging is often required, which largely increases the costs. The current study focuses on environment induced reliability problems and degradation processes. An adsorption-induced stiffening effect was observed on thin SiNx and SiCx cantilever beams in air. The resonance frequency gradually increases in time. When the cantilever is subjected to mechanical shock or large deflection, the resonance frequency suddenly drops, and then increases again. Air, increased humidity, argon rich, and nitrogen rich atmosphere influence the stiffening and the shock response behavior. The effects are explained with a surface oxidation model. The oxide layer introduces stress in the structure increasing the overall stiffness, while mechanical shocks crack the layer. Silicon resonators gather airborne particles from the atmosphere due to electrostatic charging. The mass loading decreases the resonant frequency. These mechanisms lead to unstable resonance frequency and eventually to failure of the resonant mode device. Tests in inert environment suggest, that cheap, inert atmospheric packaging will provide good performance and reliable operation.