Changes in the properties of the initial raw materials and the addition of fluxes lead to changes in the structure and metallurgical properties of pellets: reducibility, reduction strength, and softening and melting temperatures. Porosity, the nature of which determines both the mechanical properties and the rate of reduction in the blast furnace and the metallization unit, has a particular influence on the quality of pellets. It is shown that the porosity of fired pellets is formed at the stage of granulation, and during roasting it changes only due to the formation of voids from evaporation of volatile components, such as CO2 fluxes. With an increase in the dosage of fluxes, the proportion of pores from the decomposition of carbonates increases to 25% of the total mass of volatiles removed during roasting and up to 15% (relative) of the total pore space of the pellet. The formation of the pore space is determined by the gangue content and the amount of fluxes, and an increase in porosity will not necessarily contribute to an increase in the reducibility. This necessitates testing fired pellets for metallurgical properties even if there are data on their porosity. In addition, extrapolating the results of the study of pellets from iron-poor concentrates to pellets from highly-enriched concentrates is incorrect because the scale of the pore space changes.