In an effort to limit global warming to 1.5 °C by the year 2050, the Ultra Low CO₂ Steelmaking (ULCOS) program was launched, from which the HIsarna process was developed by Tata Steel in the Netherlands. This process allows us to refine low-quality iron ore and recycle one of the byproducts of steelmaking, converter slag, as a flux material. These attributes are improvements to the traditional blast furnace and offer a significant reduction in CO₂ emissions. However, research on the interaction between converter slag and low-quality iron ore at such high temperatures is limited. The main purpose of this investigation is to compare the impact of converter slag and limestone on the thermal decomposition of low-quality hematite- and goethite-bearing iron ores, particularly focusing on the conversion degree and the products of decomposition. To achieve this, the thermal decomposition of several combinations of the ores with the fluxes added at fractions ranging from 0 to 50 wt.% was first modelled thermodynamically using FactSage software. Laboratory experimentation was conducted to complement these findings, starting with thermogravimetric analysis, followed by heat treatment in a horizontal tube furnace. It was found that there was an increase in liquid phase fraction and slag content with flux addition, with the effects being more pronounced for limestone than converter slag. Additionally, the conversion degree of hematite reduced from ~10% with 0 wt.% flux to ~6.75% and ~7.60% when limestone and converter slag were added at 25 wt.%, respectively. Such a reduction in the conversion degree was attributed to the entrapment of iron in calcium-rich slag phases. Therefore, converter slag was found to be less detrimental to the thermal decomposition of low-quality iron ores, in comparison to the traditional limestone, at 1500 °C, concluding that the use of converter slag as a flux material in HIsarna is promising.