A method is provided for determination of an operating temperature in a thermally highly loaded device. Multiple individual passive measurement elements, having a measurable physical parameter depending on a constant temperature set during heat treatment, are provided. The parameter is set differently for each of the different measurement elements which are combined to form an array. In a first measurement, values of the physical parameter for the array are determined before the array is subjected to the operating temperature. The array is subjected to the operating temperature in the device and in a second measurement, values are determined for all the measurement elements in the array. First and second measurement values of the physical parameter for each of the measurement elements are compared and the temperature of the heat treatment of that measurement element whose value of the physical parameter has changed the least is taken as the measured temperature.