The microstructure formed in a steel after a partial martensitic transformation contains martensite-austenite assemblies with similar chemical composition in the two phases. In the absence of carbide precipitation, further annealing or tempering of this microstructure is believed to promote carbon partitioning from the martensite to the austenite. Experimental observations suggest that this carbon diffusion might take place in combination with migration of martensite-austenite interfaces. In this work, the effect of the martensite and austenite dimensions on the interaction between the carbon partitioning from martensite to austenite and the interface migration during annealing of martensite-austenite microstructures is analyzed. With that aim, simulations have been done by using a model in which the chemical potentials of carbon in martensite and austenite are assumed to be the same at the interface and motion of the phase interface is occurring when an appropriate driving force is present. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface.