Eutectoid steel wires are heat treated at isothermal temperatures to generate not only pearlitic microstructures but also bainite and martensite. These wires are subsequently drawn to increasing strain levels. The microstructural evolution is observed using scanning electron microscopy SEM and transmission electron microscopy TEM. Measurements of the lamellar spacing at different drawing stages reveal a thinning and a branching of the lamellae. In addition to this, the type of heat treatment influences the appearance of the lamellae and the formation of fine carbides. A numerical model was employed to compute the deformation conditions and the interlamellar spacing. The transformation and dissolution of cementite, branching of cementite and diffusion along dislocations were analysed with respect to various models from the scientific literature.