The need for in situ heat flux measurements in hot structures, used in hypersonic vehicle thermal protection system development, combustion and propulsion research, and fire testing requires that heat flux sensors are characterized over their entire operating temperature range. The in-cavity heat flux sensor calibration technique has been adapted to accommodate elevated sensor temperatures, in an effort to develop a primary calibration scheme for high temperature heat flux sensors using an existing blackbody calibration system. The new scheme has been demonstrated through the calibration of a high temperature, thermopile-type heat flux sensor. The output temperature dependence of the high temperature heat flux sensor (HTHFS) has been successfully characterized over the range of 175-960 degrees C with acceptable uncertainty limits. The calibrated HTHFS sensitivity agrees well with a theoretical sensitivity model, suggesting that the extended in-cavity calibration technique is a viable choice for primary calibration of heat flux sensors at elevated sensor temperatures. (C) 2011 Elsevier Ltd. All rights reserved.