This report presents •the first. complete microstructural and analytical electron microscopy study of Alloy AXS, one of a series 'of advanced austenitic steels developed by Maziasz and co-workers [1-7] at Oak Ridge National Laboratory, for their potential application as reheater and superheater materials in power plants that.will reach the end of their design lives .in the 1990'.s.. The advanced steels are modified with carbide forming elements such as titanium, niobium and vanadium. When combined with optimized thermo-mechanical treatments, the advanced steels exhibit significantly improved creep rupture properties compared to commercially available 316 stainless steels, 17-14 Cu-Mo and 800 H steels. The importance of microstructure in•controlling these improvements has been demonstrated for selected alloys, using stress relaxation testing as an accelerated test method. The microstructural features responsible for the imp~oved creep strengths have been identified by studying the thermal aging kinetics• of one of the 16Ni-14Cr advanced steels, Alloy AXS, in both the solution annealed and the solution annealed plus cold worked conditions. Timetemperature-precipitation. diagrams have been developed for the temperature range 600 C to 900 C and for times from 1 h to 3000 h. Particle size measurements have shown that the MC precipitates formed on matrix dislocations resist coarsening for up to 3000 h at 900 C.