Abstract. Our study of 10 M - class solar flares observed by the SOXS mission reveals breakthrough nature of multi-thermal X-ray emission in 6-20 keV with temperatures of 15-50 MK. The multi-thermal power-law index gamma and the flare plasma cooling time vary between 2.7 - 4.3 and 22 - 310 s respectively. Further the break-energy point between thermal and non-thermal emission is found to vary between 17 and 22 keV.