An optical fibre sensor system capable of detecting contaminants (e.g. particles, inorganic or organic species) in water and other fluids is reported. In this article experimental results are presented for a single optical fibre sensor located at a distance of 150 m from the transmitter/receiver of the system. The fibre is addressed using an optical time domain reflectometer (OTDR) in order to achieve the spatial resolution (along the fibre length) necessary for this investigation. Novel signal processing techniques involving artificial neural networks and pattern recognition have been applied to the signals arising from the sensor in order to allow cross-sensitivity effects, e.g. from fouling due to calcification, to be extracted from the real measurand, e.g. alcohol content.