A novel mercury ion selective electrode has been constructed to have strongly coordinating macrocyclic ligand binding site covalently attached to polythiophene film using 1-1,4,10-trioxa-7,13-diazacyclopentadecane-1-thiophenylmethane (TDCD-thiophenylmethane) as a specific chelator. In amperometric mode, the current is proportional to mercury (II) concentrations down to 2 ppb. Reversible cyclic voltammetric waves show that both mercury (I) and mercury (II) are stabilized by the ligand. Monomethyl mercury and vapor phase elemental mercury are also electrochemically determined. Dissolved oxygen and heavy metal cations show no significant interference even at ten times the mercury concentration. The polymer surface shows robust performance for two years or longer. Fabrication strategies are being developed to optimize the probe performance. The variables under investigation include substrate surface roughness, the ratio of derivitized thiophene (TDCD-thiophenylmethane) to underivatized thiophene in the surface polymer, ratio of thiophene with two chelating rings to thiophene with only one, number and thickness of electrodeposited polymer layers, composition of polymer layers, and number of -CH2- units separating the chelating ring from the thiophene monomer units. The probe exhibits linear response to mercury concentration, and detects mercury in several forms.