Recent advances in heat flux measurement have resulted in the development of a robust thermopile heat flux sensor intended for use in extreme thermal environments. The High Temperature Heat Flux Sensor (HTHFS) is capable of simultaneously measuring thermopile surface temperature and heat flux at sensor temperatures up to 1000 degrees C. The need for high temperature heat flux calibration of the HTHFS has resulted in the development of a new wide angle radiation calibration system, which operates with the sensor at elevated temperatures. The temperature dependence of the sensor output over the range of 100-900 degrees C has been successfully characterized with acceptable uncertainty limits. The calibrated HTHFS sensitivity agrees well with a theoretical sensitivity model, suggesting that the primary cause for the sensor's output temperature dependence is due to the change in thermal conductivity of the sensor elements with temperature. (C) 2010 Elsevier Ltd. All rights reserved.