Many ligands such as 1-nitroso-2-naphthol (NN), 4-(2-pyridylazo)resorcinol (PAR), 2,4-dinitrosoresorcinol (DNR) and 1-(2-pyridylazo)-2-naphthol (PAN) have been investigated in the development of optical sensors for heavy metal ions found in aquatic environments which include Co2+, Cu2+, Ni2+, Fe3+, Cd2+, Zn2+, Pb2+ and Hg2+. The reagents were immobilised by physical adsorption on to polymeric materials such as XAD-4, XAD-7 and Dowex ion-exchange resins, and were found to exhibit chromic characteristics in the absence and presence of metal ions in solution. Reflectance measurements are used here. Parameters such as pH, measurement wavelength and reagent concentration have been optimised for a given metal ion, and the reversibility/regenerability characteristics of the sensing reagent have been studied together with the limit of detection and the concentration range producing linear response for the metal ion. (C) 1998 Elsevier Science S.A. All rights reserved.