This study concerns the evaluation of ionophoric properties of a functionalized calix[4]arene, named 5,17-bis(diphenylphosphinoyl)-25,26,27,28-tetrapropoxycalix[4]arene (Calix), by its application as a sensing material in an erbium(III)-PVC membrane potentiometric sensor. A membrane formed by 30% PVC, 65% dioctyl phthalate (DOP) as plasticizing solvent mediator, 2% sodium tetraphenyl borate (NaTPB) as ion excluder, and 3% of Calix as ionophore shows the best performance of the sensor. The prepared sensor presents a nearly Nernstian response (a slope of 20.0 +/- 0.5 mV/decade) over a concentration range of 1 x 10(-7) -1 x 10(-3) M of erbium ions, with a limit of detection of 3.8 x 10(-8) M. Its potential response was independent of pH variation in the relatively wide range 4.5-6.5. The dynamic response time of the electrode to achieve a steady potential was found to be about 7 s. The selectivity of the sensor towards erbium ions with respect to several mono-, di-, tri-, and tetravalent metal ions was examined. The prepared sensor can be used for 3 months without considerable divergences in potential response. The performance of the sensor was examined as an indicator electrode for complexometric titration of erbium solutions by EDTA.