Abstract: In order to clarify the cause of failure in waste heat boilers with vertical U-shaped tubes, variation of wall temperature along the bend under thermal conditions was investigated in an electrically heated high pressure water loop experimental installation. The water loop consisted of a 25 mm i. d. stainless steel tube with a 180° bend of radius 700 mm operating under the conditions of P=106×105Pa, G=800-2000 kg/m2·s and q = 80-330kW/m2·In the tests, the circumferential wall temperature profile along the bend was found to increase from the middle of the downward section of the tube to 15°of the upward section, at low mass velocities and low steam qualities, reaching a peak value at (?)=75°. The temperature difference, △t, between the inner side and outer side of the bend was found to vary with mass velocity, steam quality and heat flux, increasing abruptly to a peak value at low steam qualities. Variation of △t was noted to reflect the influence of flow pattern transitions. As may be anticipated, the flow regime belongs to stratified flow for low mass velocities and low steam qualities. In this regime, slow-moving vapor, packets on the inner wall of the bend, and hence low heat transfer rates may be expected to lead to large temperature gradients. For bigher steam qualities, inter-mittent two-phase flow may be expected to reduce the temperature difference. Finally, appearance of liquid film around the tube wall in the annular flow regime may be anticipated to result in considerable suppression of the temperature difference.