A new solvent polymeric membrane sensor based on tetra(p-chlorophenyl)porphyrinato manganese (III) acetate is described which demonstrates excellent selectivity toward the triiodide ion. The electrode has a linear dynamic range between 1.0 ${ imes}$ $10^{-2}$ M and 7.0 ${ imes}$$10^{-6}$M with a Nernstian slope of $-59.6{pm}1$ mV per decade and a detection limit of 5.0 ${ imes}$$10^{-6}$M. The proposed sensor revealed good selectivities for triiodide over a wide variety of other anions and could be used in a pH range 2-9. The electrode can be used for at least two months without any considerable divergence in potential. It was applied as indicator electrode in potentiometric titration of the triiodide and As(III) ions.