A novel polyvinyl chloride (PVC) membrane sensor that is highly selective to Fe3+ ions was prepared by using 5-chloro-3-[4-(trifluoromethoxy) phenylimino] indolin-2-one (CFMEPI) ionophore. The sensor exhibits a Nernstian response for Fe3+ ions over a wide concentration range (1.0 x10(-2) -1.0x10(-6)M) with a slope of 46.7 +/- 0.5 mV per decade. The sensor has a response time of 20s and can be used for at least 3 months without any measurable divergence in potential. It was concluded that the sensor response was pH independent in the range of 4.0-8.61. The sensor has some advantages such as short analysis time, particularly high selectivity towards iron (III). The sensor was used successfully for direct determination of Fe3+ in several synthetic and real samples with satisfactory results.