In-pile electrochemical measurements were performed in order to investigate the effect of radiation on the electrochemical corrosion behaviour of AISI 304 and AISI 316 in PWR primary water (400 ppmB and 2 ppmLi) at 300 ◦C. All measurements were performed on both an in-flux and an out-of-flux three-electrode electrochemical cell, each containing a high temperature reference electrode. In this paper we focus on the EIS measurements that have been carried out. The quality of these EIS data varied from good to rather poor as the electrical feed through degraded in time due to the harsh test conditions, i.e., radiation, high temperature and high pressure solution with boric acid and lithium hydroxide. The Nyquist diagrams do change in time, but still reflected the phenomena that took place at the metal–metal oxide-solution interface. This could be modelled with an equivalent circuit where the isolation resistance was placed in parallel with the actually equivalent circuit for the working electrode impedance. Measured and simulated Nyquist plots were qualitatively in good agreement and show a difference in the oxide layer structure of the in-flux and out-of-flux electrodes.