High efficiency steam power plants are planned to operate at temperatures higher than 700°C and at a pressure of up to 350 bar. Due to this increase of the steam parameters, Ni-based alloys are required for constructing these plants. Materials testing - based on appropriate manufacturing and design criteria - is necessary in order to have a reliable data base of the relevant design characteristics. Additionally, a better understanding of the specific material behaviour under service like loading conditions for the evaluation of possible damage mechanisms is essential. This paper describes research on the behaviour of thick-walled power plant components made of Alloy 617 mod. and Alloy 263. Results from basic qualification programs with standard specimens including welded joints show the applicability of the materials. Results from creep rupture tests of base material and welded joints and microstructural investigations to obtain information on precipitations and dislocations in the virgin and aged conditions are presented. Information on the influence of chemical composition on creep rupture was obtained by analysing creep rupture data sets.