During irradiation tests at high temperature failure of commercial Inconel 600 sheathed thermocouples is commonly encountered. As instrumentation,in particular thermocouples are considered safety-relevant both for irradiation tests and for commercial reactors, JRC and THERMOCOAX joined forcesto solve this issue by performing out-of-pile tests with thermocouples mimicking the environment encountered by high temperature reactor (HTR) in-core instrumentation. The objective was to screen innovative sheathed thermocouples which would consecutively be tested under irradiation. Two suchscreening tests have been performed in high temperature environment (i.e. temperature in the range 1100?1150?C) with purposely contaminated heliumatmosphere (mainly CH4, CO, CO2, O2impurities) representative for high temperature reactor carburizing atmospheres. The first set of thermocouplesembedded in graphite (mainly conventional N type thermocouples and thermocouples with innovative sheaths) was tested in a dedicated furnace atTHERMOCOAX lab with helium flushing. The second out-of-pile test at JRC with a partly different set of thermocouples replicated the original test forcomparison.Performance indicators such as thermal drift, insulation resistance and loop resistance were monitored. Through these long-term screening teststhe effect of several parameters were investigated: niobium sleeves, bending, diameter, sheath composition as well as the chemical environment. SEMexaminations were performed to analyze local damage (bending zone, sheath).The present paper describes the two tests, sums up data collected during these tests in terms of thermocouple behavior and describes further instru-mentation testing work with fixed point mini cells for qualification under irradiation.