Local corrosion phenomena, mass and interstitial element transfer rates depend on the difference of chemical activity in lithium or sodium and in structural materials of a reactor circuit. Results are commented related to the compatibility tests of solid electrolytes (for oxygen cells) in lithium. Preliminary results are reported of EMF-measurements using different solid electrolytes (for oxygen meters) with either lithium or lithium-lead. The output voltage of an oxygen meter with ThO//2 minus 7. 5 wt% Y//2 O//3 solid electrolyte and air as a reference electrode, which lasted for 14272 h in a dynamic sodium system, is presented as a function of cold trap temperature.