Dithizone and its dicarboxylic acid derivative (DDz) have been immobilised onto gold films by both chemical attachment and incorporation into spin-coated polymer films. The modified gold films were characterised by ellipsometry. Surface plasmon resonance (SPR) was employed to monitor changes in the optical properties of the dithizone films in the presence of aqueous transition metal ions. The dependence of applied potential on the SPR response in the presence of metal ions has been studied in situ. It is shown that the technique can be used for the determination of metal ions in water.