Abstract: As part of efforts to reduce anthropogenic CO₂ emissions by the steelmaking industry, this study investigated the direct reduction of industrially produced hematite pellets with H₂ using the Doehlert experimental design to evaluate the effect of pellet diameter (10.5–16.5 mm), porosity (0.36–0.44), and temperature (600–1200°C). A strong interactive effect between temperature and pellet size was observed, indicating that these variables cannot be considered independently. The increase in temperature and decrease in pellet size considerably favor the reduction rate, while porosity did not show a relevant effect. The change in pellet size during the reduction was negligible, except at elevated temperatures due to crack formation. A considerable decrease in mechanical strength at high temperatures suggests a maximum process operating temperature of 900°C. Good predictive capacity was achieved using the modified grain model to simulate the three consecutive non-catalytic gas–solid reactions, considering different pellet sizes and porosities, changes during the reaction from 800 to 900°C. However, for other temperatures, different mechanisms of structural modifications must be considered in the modeling. These results represent significant contributions to the development of ore pellets for CO₂-free steelmaking technology.