Thermal joining of newly developed, high alloy CrMnNi cast steels showing the TRansformation Induced Plasticity (TRIP) and/or TWinning Induced Plasticity (TWIP) effect is an essential prerequisite for its industrial application as structural components. It is particularly appropriate to use high energy beam welding processes like laser and electron beams for this task because of their characteristically precise and short time-interval energy inputs, which result in minimized thermal loading of the base material. Welding by electron beam (EB) proved to be particularly advantageous because of its rapid beam deflection and, furthermore, the excellent possibilities exhibited by beam guidance in achieving optimized welding results (geometry, prevention of pores, and cracks). The present paper deals with initial investigations of EB-welded high alloy TRIP/TWIP cast steels. Using available EB equipment, a maximum welding depth of 26 mm was achieved. Excellent weldability for all investigated materials was demonstrated. The crack and pore-free welding seams showed a slight increase in hardness of 50 HV0.3 compared to the base material. Tensile tests carried out on welded and unwelded specimens showed no significant decrease in their mechanical properties. Rupture of the welded specimens occurred outside of the welding seam.