Abstract: The sensor consisted of a polished Ag disc (0.9 cm o.d.) which was fixed to a small piece of polyethylene tubing (0.5 cm long, 1 cm o.d.) which was in turn fixed to the inner surface of a glass tube (7 cm long, 1 cm i.d.). The inner surface of the disc was soldered with Ag epoxy to the Cu core of a coaxial cable. A mixture consisting of 10 mg harmine or harmaline tetraphenylborate or reineckate, 352 mg dioctylphthalate or tributylphosphate, 187 mg PVC powder and 7 mL THF was prepared and packed into the sensor body. The solvent was allowed to evaporate to form a coating. The sensor was used in a FIA system using 0.1 M phosphate buffer of pH 4.6 as the carrier solution (5 ml/min). Potentiometric measurements were made relative to Ag/AgCl reference electrode. The response was linear from range 10 µM to 10 mM with a detection limit of 2 µg/ml. The effects of pH and the presence of foreign compounds were investigated.