The variation in thermoelectric power with stress of platinum and platinum/13% rhodium wires with junction temperatures of 20 and 550°C has been found to be -47 pv degc⁻¹ kg⁻¹ cm⁻² and - 12 pv degc⁻¹ kg⁻¹ cm⁻² respectively. These results enable the thermal e.m.f. hysteresis loop observed with some metal-sheathed mineral-insulated thermocouples to be explained and the maximum discrepancy produced by this loop has been calculated at 23 μv at 550°C. An experimentally determined result on a single thermocouple gave 14 μv. The mechanical hysteresis associated with the loop may lead to fatigue failure, and it is concluded that for a reliable thermocouple of this type the coefficients of expansion of the sheath and thermocouple materials must match.