The combined use of a X-ray diffractometer and a high temperature chamber allows in-situ determination of microstructure during heat treatment. X-ray diffractograms are recorded during tempering of high speed steels. The Rietveld method is used in connection with size-strain analysis. Lattice parameters of austenite and martensite are utilized for estimating the change of carbon content that has massive influence on the secondary hardening. Nevertheless, it was shown that the heat treatment of deep-frozen high speed steel cannot be deduced a priori from the applied tempering process under conventional quenching conditions. The differently conditioned microstructures are based on different phase amounts and microstrain-situations.