We present spectral analysis methods suitable for diagnostics of flaring plasma from RESIK spectra. RESIK is the uncollimated bent crystal spectrometer aboard the Russian CORONAS-F solar mission. It collected many flare and active region spectra in the wavelength range 3.3 Å–6.1 Å, where strong emission lines of Si, S, Ar, and K are present. Based on a careful instrument calibration the absolute fluxes in the individual spectral lines have been obtained. These fluxes have been used for determination of a set of thermodynamic parameters characterizing the emitting plasma and for studies their time behavior during selected flares.