The effort to minimize CO2 emissions leads the existing integrated steel plants to implement alternative biomass-based fuels that dispose of equilibrium carbon balance. The fuel is a key factor in the iron ore sinter production, so it is essential to know its impact not just on mechanical properties of the finished sintered ore but also on the mineral composition as the mineral phases together determine all observed sinter properties. For this purpose the samples prepared by replacing a part of coke breeze with charcoal or walnut shell substitute were subjected to the observation under the light microscope, also using etching, to the phase identification by chemical EDX analysis on the scanning electron microscope and to the phase composition quantification by X-Ray diffraction analysis. The studied microstructure areas in the vicinity of the pores left by fuel grains were neither characterized by different phases nor by changed chemical composition of these phases even thought mineral matter of the used fuels were substantially different in terms of the chemical composition. The only feature of the burned substitute fuels were ash particles arranged in characteristic shapes. The main reason of variation in ratios of respective mineral phases of samples appeared to be thermal conditions that were reflected in the content of unreacted non-ferrous phases. Coke substitution in the sinter mixture has no negative impact on the phase composition of the produced sinters, which confirms the prospective use of biofuels in the sintering process.