A theoretical basis for the application of a two-thermocouple probe, comprised of two fine-wire thermocouples of different diameters, to carry out fluctuating temperature measurements is established with the aid of the method of least squares. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures has been realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, there is no need to introduce a so-called time-constant ratio into the reconstruction scheme. Consequently, the essential drawbacks in the two-thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method of correcting for heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the original reconstruction scheme with little modification. The scheme developed has been tested in a fluctuating temperature held with a diffusion flame formed in a two-dimensional wind tunnel. It is shown that the present scheme is successful and even allows study of the characteristics of time-constant fluctuations in high-temperature turbulent flows.