Surface plasmon resonance (SPR) is a powerful technique for direct sensitive (bio)chemical detection. This phenomenon can be used to measure the refractive index of either bulk chemical samples or chemically sensing thin layers. In this work, a surface plasmon resonance fibre optic sensor has been developed. A 50 nm thick silver film is deposited by thermal evaporation onto the silica core of the optical fibre. To protect silver from oxidation, the evaporated silver film was covered with self-assembled monolayers (SAMs) of long-chain alkanethiols (1-octadecanethiol), To characterize these SAMs, silver films evaporated onto macroscopic glass surface as test samples and several techniques such as contact-angle measurements (sessile drop method), X-ray photoelectron spectroscopy, and atomic force microscopy were used. Such a modified surface showed a high contact angle with water which remains stable after a one month stay in aqueous media. Similarly, a SPR fibre optic sensor showed no ageing problem, when kept in the same conditions.