Nowadays, many systems to obtain the value of concentration of contaminants dissolved in water have been developed. These systems use to take a sample from the water (sea, rivers or lakes), make a preparation and, finally, analyse the prepared sample. This process spent a lot of time and needs very specialized personnel for doing it. In this paper, an on-line system to obtain the value of concentration of several contaminants is presented. It allows us to have a permanent control of concentration values. The measurement process uses the fluorescence characteristics of these contaminant substances and it can be used for many petroleum-derived substances.