The influence of Re and Ru on creep rupture strength has been investigated using a new in-house designed alloy-series comprising 9 experimental nickel-base superalloys with stepwise increased Re and Ru additions. The presented creep data reveals a significant increase in creep rupture strength by additions of Re. For additions of Ru an increase of creep rupture strength can only be found for low Re contents. The present article, which is part of an extensive and systematic investigation on Re and Ru influences, shows, that an improved creep resistance by an influence of Re and Ru on the γ’-solvus temperature is rather improbable. Likewise, the influence of Re and Ru on liquidus temperature is not expected to play an important role. However, the creep rupture strength is suggested to be highly modified by γ/γ’-microstructure changes.