A new Fe(3+) ion-selectivemembrane electrode based on 6-ketomethyl phenanthridine (6-KMPT) and 1-(4-dimethyl aminophenyl)-2-(5H-phenanthridine-6-ylidene)-ethanone, which were incorporated in a plasticized polyvinyl chloride (PVC) membrane, is described. The optimized membrane demonstrated a linear dynamic range of 5.2 x 10(-6)-1.0 x 10(-2) M, with a near Nernstian slope of 19.5 +/- 1.5 mV per decade and a detection limit of 1.2 x 10(-6) M. The electrode showed high selectivity for Fe(3+) ions in comparison to other metal ions. The effect of membrane composition on the potential response of the electrode was studied. The best performance was observed for the membrane comprised of electroactive material (6-KMPT), dioctyl phthalate (DOP) as plasticizer, and PVC in the optimum ratio of 5:67:28% w/w. The pH working range of the sensor was 1.2-3.0. The proposed Fe(3+) sensor was successfully used for determining the presence of iron in pharmaceuticals and the results were in good agreement with the value obtained using the atomic absorption spectroscopic method.