A conductometric technique for the determination of third phase, a phase formed by splitting of organic phase during malfunction in operation, during reprocessing of fast reactor fuel using a high resolution conductivity monitoring instrument with pulsating sensor developed in-house was tested. Initial studies were carried out in laboratory using uranous salt solutions (U 4+)in various synthetic samples containing different aqueous (A) to organic (O) phase ratios (A/O). Results show that there is appreciable increase in conductivity of third phase solution compared to the other organic phase (∼100 to ∼300 times higher). Such a large change in conductivity in third phase with respect to the other organic phase can be used as a deciding parameter to give first hand information about the occurrence of third phase. © 2012 Akadémiai Kiadó, Budapest, Hungary.