The chemical instability of the gallium arsenide surface poses a serious limitation to its use under ambient conditions, such as in air or aqueous solution. It is shown that both bare GaAs and GaAs coated with self-assembled monolayers of organic alkane-thiols are continuously etched in aqueous environments, which limits their use as biosensor devices in physiological environments. A corrosion protection material having long-term stability ("chemical passivation") and biocompatibility ("biological passivation") with the GaAs surface was found to be interfacial layers of polymerized organic mercaptosilanes a few tens of nanometers thick. The mercaptosilanes not only provided a nearly perfect corrosion protection of GaAs in water, but they also have the potential to introduce chemical groups that allow easy, further surface functionalization. Characterization of the inter-facial polymer layer and its protective role was done by atomic force microscopy (AFM), ellipsometry, contact angle measurements, and atomic absorption spectroscopy (AAS). The interfacial polymer layers fully suppressed the release of arsenic into the electrolyte buffer and also provided an adhesion-promoting interface, which allowed the cultivation of electrically excitable cells, normal rat kidney (NRK) fibroblasts, on GaAs. The electrical performance of GaAs and GaAs/InGaAs heterostructures in water was monitored via cyclic voltammetry and the IU characteristics of field-effect channels. GaAs was significantly stabilized by the insulating polymeric surface coatings, even under moderate electrochemical loads. These findings are promising for, e.g., the implementation of GaAs technology in future cell-semiconductor hybrids.