Pt and its alloys have important applications in the field of catalysis or in the jewellery industry. Especially in the latter case sufficient strength is necessary to ensure wear resistance. We thus explore here the potential of severe plastic deformation on the strengthening behaviour of two Pt-Ru alloys. Depending on the Ru content and the deformation conditions hardness values of up to 6.5 GPa could be achieved. A further hardness increase to more than 7 GPa can be obtained by adequate recovery annealing. Indentation with different tip geometries indicates that the specimens undergo significant strain softening after the heat treatment. The results further show that the anneal hardening effect occurs only below a specific threshold grain size, with the extent increasing towards smaller grains. This can be rationalized by the increasing role of grain boundaries for mechanical properties at the submicron scale. Considering different possible origins of the hardness increase and bearing in mind that indentation techniques sample the strength at rather large plastic strains, the results seem to be best explained by annihilation of lattice dislocations and grain boundary defects, complicating the absorption of dislocations at the grain boundaries after the heat treatments.