A coated graphite-epoxy iron(III) ion-selective electrode, based on the ion-pair between [Fe(oxalate)3]3−anion and tricaprylylmethylammonium cation (Aliquat 336) in a poly(vinylchloride) (PVC) matrix is constructed. A thin membrane film of this ion-pair, dibutylphthalate (DBPh) in PVC was deposited directly onto a PerspexRtube containing a graphite-epoxy conductor substrate attached to the end of a glass tube. The effect of membrane composition (ion pair, DBPh and PVC), oxalate concentration, pH and some cations and anions upon the electrode response is investigated. The electrode shows a linear anionic response to E vs. log[Fe3+] in the iron(III) concentration range from 2.9×10−6to 10−2mol/L, and a slope of −18.7±0.5 mV/dec, at pH working range 2-8 and 0.3 mol/L oxalate concentration. Variation in the potential of about ±2 mV was observed during a working day of 7-8 h. The response time was less than 5 sec and the lifetime of this electrode was superior to one year (over 1500 determinations by each polymeric membrane), with a practical detection limit of 2.1×10−6mol/L. Application of this electrode for iron(III) determination in biotônico sample (Brazilian tonic formula) is described.