Defining the proper conditions for sintering of steels containing oxidation-sensitive elements requires a deep understanding of the overall gas-gas and solid-gas reactions taking place especially during the heating stage of the sintering process. In this work, Mn and Si have been introduced in sintered steels in the form of a master alloy powder specifically designed to form a liquid phase. The effect of sintering atmosphere (Ar, H2 or N2-10H2) and carbon content on both reduction/oxidation reactions and dimensional stability were studied by advanced thermal techniques. Two different types of base powders (plain iron and Cr-prealloyed steel) were considered with the aim of understanding the effect of chemical interaction between base powder and master alloy. The results show how heterogeneity in chemical activity between the components of the mix modifies the overall reduction reactions, and how dimensional changes are importantly affected by the presence of reducing agents: either H2 from the atmosphere or carbon introduced as graphite.