The real-time detection of phenols in groundwater using a fibre optic fluorescent sensor system is discussed. Two sensor designs are discussed, modelled and experimentally evaluated. One, consisting of a single source fibre and nine ‘return’ fibres used with a deuterium lamp source, double monochromator and photomultiplier detector is compared to a dual fibre device which is excited by a frequency quadrupled Nd-Yag laser source, narrow pass band optical filter and photomultiplier detector. The latter system has a reference channel to compensate for temporal variations in laser output optical power. Phenol resolution of less than 1 ppb was achieved with the laser based sensor operating over a source-sample distance of 15 metres.