In this study, a very sensitive and highly selective irreversible optical chemical sensor (optode) for mercury ions was described. The sensing scheme was based on the interaction of Hg (II) with a newly synthesized fluoroionophore; chloro phenyl imino propenyl aniline (CPIPA) in plasticized PVC membrane. The sensor membranes were tested for the determination of mercury ion in aqueous solutions by batch and flow-through methods. The optodes allow determination of Hg (II) in the working range of 1.0 x 10(-9)-1.0 x 10(-5) M with a detection limit of 4.3 ppb. The sensor exhibited excellent selectivity for Hg (II) with respect to several common alkali, alkaline earth and transition metal ions. The association constant of the 1:1 complex formation for Hg (II) was found to be K(a) = 1.86 x 10(5) M(-1). The CPIPA exhibited high fluorescence quantum yield, long excitation and emission wavelength and high Stokes' shift values in the solid matrix which makes it compatible with solid state optics.