[en] An ion microprobe has been used to identify silicon as the embrittling agent responsible for the tensile failure of a Pt-30Rh-8W alloy. Silicon was found to be segregated on the alloy, and to have a concentration gradient along them from the outer surfaces toward the interior of the sample. The source of the silicon contaminant was found to be the alumina mounting apparatus. This is consistent with the statement of DARLING et al. that contamination of Pt/Pt-Rh thermocouples is representative of the impurities in refractory mounting shields rather than of the refractory itself. (T.G.)