A hot plasma can be analysed by means of soft-X-ray fluxes using various absorbers. In the first part of the paper, the method of calculation of the flux as a function of Te, Zeff, ne and their profiles is given. The problem of relativistic corrections in the evaluation of the Gaunt factor is also considered. As an example, in a 200-kA discharge in TFR, the measured fluxes are compared to those calculated for a Maxwellian electron distribution function. From this comparison, the electron temperature is determined and information on the impurity content in the plasma is obtained, in agreement with other independent measurements.