Abstract—Isothermal and thermal‐mechanical strain fatigue tests were conducted in air on representative service alloys; a 1 CrMoV steel, and two batches of 316 stainless steel. Data was obtained for thermal‐mechanical in‐phase and out‐of‐phase cycles, and also for isothermal tests at the maximum, minimum, and mid‐temperature of the thermal‐mechanical cycle. Dwell periods were also incorporated in the cycle to assess their effects.A comparative evaluation has been made on the basis of the materials' cyclic stress response. In general, the results have shown that the thermal‐mechanical strain cycling tests cause a large increase in stress range over those tested under isothermal conditions at maximum temperature. In addition, mean stress and strain offsets were developed in continuous cycle thermal‐mechanical tests, whereas negligible offsets occurred in isothermal tests.It appears that the response of the materials could not always simply be explained by reference to the temper