Basic research considering the minimum achievable grain size of severe plastic deformation (SPD) materials is often performed with high purity metals. However, a literature study reveals large discrepancies in the microstructural size of high purity nickel processed by SPD. The focus of this work is the influence of small impurity concentrations on the resulting saturation microstructure. The microstructure of SPD nickel was systematically investigated using different purities ranging from 99.79 wt% up to 99.99 wt%. The materials were deformed by high pressure torsion (HPT) at different temperatures from liquid nitrogen temperature (-196°C) up to 400°C. It was found that carbon concentration is the governing element in achieving the finest microstructures and the highest strength. Therefore, further experiments with well defined carbon doped samples were performed. By changing the carbon content from 0.008 wt% up to 0.06 wt% tensile strength in the saturation regime increases by more than 700 MPa. It will be shown that even small variations (<0.01 wt%) lead to significant changes in ductility and tensile strength values.