A novel fluorescence optical sensor for sensitive and selective determination of Hg2+ ions in aqueous solution has been prepared. The sensing system is made of a novel unsymmetrical tetradentate Schiff base namely N-(2-hydroxyphenyl)-N'-(2-mercaptophenyl)-o-phthalylidene (HMP), as a neutral Hg2+-selective fluoroionophore immobilized in a plasticized PVC membrane containing potassium tetrakis-(4-chlorophenyl) borate (KTpClPB) as a lipophilic anionic additive.