With an adequate correction, thermocouples can be used for the measurement of high gas temperatures within enclosing walls of different temperatures. Considering heat radiation between thermocouple and surroundings, Fischer developed a two-sensor method for the case with only one unique surrounding surface temperature, which is widely used for this kind of correction. The modified two-sensor method derived in this paper is applicable to the case with different surface temperatures of the surroundings, if inter-reflections between the surrounding surfaces may be neglected. Fischer's correction may thus be considered as a special case of this modified two-sensor method.