In this diploma thesis the AerMet® 100 alloy was investigated. This alloy has a chemical composition of Fe-13,4%Co- 11,1%Ni- 3,1%Cr- 1,2%Mo- 0,23%C (mass per cent) and belongs to the group of the “ Secondary Hardening High Co-Ni Steels“. The microstructure consists of a predominantly nickel-martensitic matrix, in which (Cr,Mo)2C carbides are formed through aging. The special properties of this alloy are fracture toughness values of more than 120 MPa√m with corresponding tensile strengths of 2000 MPa. The aim of this work was to characterize the constitution, the structure and the mechanical properties of the AerMet® 100 alloy. Therefore test-alloys with different cobalt content were produced. With these test-alloys the influence of the cobalt content on the hardness and on the structure depending on the aging temperature was assessed. Further the AerMet® 100 alloy itself was analysed regarding the structure. Additionally, to investigate the mechanical properties of the AerMet® 100 alloy tensile, notch impact and fracture toughness tests were performed. The results were compared with the mechanical properties of a Maraging Steel of the trademark Boehler W720 and with the literature values for the AerMet® 100 alloy.