Towards development of electrical conductivity based sensors for online detection of formation of third phase during reprocessing of Pu rich spent nuclear fuels, laboratory studies were carried out using U4+ solutions in various experimental conditions. Third phases were generated by extracting U4+ from nitric acid medium by 1.09 M TBP at different A/O (A: aqueous, O: organic) ratios. The results of this study indicate that the third phase is nearly 100-300 times more conducting than lean organic phase and saturated phase. The higher conductivity of third phase as compared to that of other two phases is explained based on the principle of reverse micelle formation and charge movement between the micro emulsion globules by percolation phenomenon. © 2012 Akadémiai Kiadó, Budapest, Hungary.