The sensitivity of thermo-electric power (TEP) measurement to detect the undesired alpha-alpha' phase separation occurring in Cr-rich oxide dispersion strengthened steels was investigated. TEP values were found to increase with time of exposure to 475 A degrees C-the maximum temperature of the miscibility gap in Fe-Cr alloys. Exposure to 650 A degrees C did not induce any changes in the TEP value. By examining the bend contours in dark-field TEM images, alpha' precipitates were evidenced in aged PM2000. Using the Fe-XCr-YAl model alloys and X-ray photoelectron spectroscopy, the changes in TEP were traced back to the depletion of Cr from the matrix, caused by the formation of the Cr-rich alpha' phase. By quantifying the effect of Cr content on the TEP value of model alloys, it was estimated that following 1000 h of aging, the Cr concentration in the alpha matrix of PM2000 alloy was reduced from 20 to similar to 13.5 at.%.