In this work the performance of an optoelectronic sensor system able to measure the concentration of organic analytes, in aqueous environment was analyzed. Syndiotactic polystyrene (sPS) in the nanoporous crystalline δ form was here used as sensitive polymeric layer due to its high sorption properties towards chlorinated and aromatic compounds. In particular, a thin semi-crystalline sPS film with crystalline nanoporous δ form was cast on the cleaved end of a standard fiber optic and measurements of reflectivity as the analyte was sorbed by the polymer were carried out allowing on-line detection of even very low concentrations (in the range of few ppm) of chloroform and toluene in water. The refractometer performance was compared with that of a classical resonant quartz-crystal microbalance sensor (QCM), using the same polymer as sensing element.