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Electrochemical potentiokinetic reactivation (EPR) testing provides quantitative detection of small degrees of sensitization. We have used double-loop (DL-EPR) testing, a method which has been characterized for use on austenitic stainless steels, to measure sensitization resulting from aging or from welding of PH 13-8 Mo martensitic stainless steel. Aging at either 500 deg C or 620 deg C results in an increase of the reactivation current density. The 500 deg C treatment promotes preferential susceptibility to corrosion along prior austenite grain boundaries, and the 620 deg C treatment promotes preferential susceptibility along martensite interlath boundaries. A narrow band in the heat-affected zone of autogenous weldments also undergoes localized corrosion during the reactivation scan. Increased reactivation current density is likely caused by classic Cr-depletion resulting from carbide precipitation. (ERA citation 12:013778)