Pellet structure determines metallurgical properties at both the macro and mineral/elemental levels. The purpose of this paper is to analyse the change in porosity of iron ore pellets during their life cycle, from the pelletiser to the fully reduced state. A model based on the following assumptions was used as a research hypothesis: 1. The primary porosity of pellets is established during the green pelletising (balling) stage, with the pore space partly filled with water and partly with air. When the pellets are dried, the water evaporates and the pore space is filled only with air. The geometric dimensions of the pellets remain unchanged. 2. During high temperature roasting and sintering, the volume of the pore space changes due to the removal of gaseous products of the chemical reactions of decarbonisation, dehydration, oxidation and sintering. The increase in porosity is proportional to the amount of combustion products removed, and the volume reduction during sintering generally does not exceed 2 - 5%. 3. the reduction of pellets increases their porosity by a number of parallel processes: volume increase (so-called ‘swelling’) and reduction of the mineral part of a pellet by reduction of iron oxides. Finally, the porosity of reduced pellets is 2-3 times higher than that of oxidised pellets (up to 46...60%). Analysis of the experimental data confirmed the validity of this model, which can be used to calculate pellet porosity. It has been shown that the porosity of pellets undergoes a predictable transformation during their life cycle and that oxidative roasting can lead to both an increase and a decrease in porosity, depending on the composition of a mixture. Reduction of pellets always results in an increase in porosity by a factor of 2 to 3.