The alloy system Fe-Co-Mo can be hardened by an intermetallic phase. The hardening behaviour is similar to precipitation hardened aluminium alloys. The carbon-free alloys are comparatively soft in the as-quenched martensitic condition and are hardened during subsequent heat treatment at moderate temperatures. These properties can be related to a martensitic matrix with a high dislocation density and nm-sized precipitates. Changing/extending the alloying concept affects formation, process of growth and type of the intermetallic phase responsible for hardening of the martensitic matrix. In this work, the influence of Cr, Nb and V on the precipitation behavior of an Fe-25 m%Co-15 m%Mo was investigated by atom probe, differential scanning calorimetry, scanning electron microscope, small-angle x-ray scattering and x-ray diffraction. Although the principial hardening mechanism (precipitation hardening) did not change due to the addition of the mentioned alloying elements a significant inluence on the precipitation behavior was revealed by the conducted experiments. Both, maximum hardness as well as the temperature where maximum hardness is reached, are shiftet to lower values. This phenomenon could be related to an accelerated formation of Mo-rich cluster at low aging temperatures because of the addition of mentioned alloying elements. Moreover, by means of diffraction experiments a change in crystallographic structure of the intermetallic precipitates, responsible for the hardnesss increase, could be established.