The use of 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid [C 4 mim] [PF 6 ] as additive in a Pb(II) plastic membrane electrode increased significantly the selectivity of the sensor. Atomic force microscopy and depth profiling X-ray photoelectron spectroscopy were applied to investigate the compositional and morphological changes of the surface and bulk of the membrane during the conditioning process of the electrode.