Electron microscopy and atom-probes techniques, including 3D (three-dimensional) atom-probe, were applied to the investigation of grain-boundary (GB) microchemistry and interfacial segregation phenomena in nickel base superalloys, Astroloy. Diffraction patterns and EDX microanalyses exhibit the presence of intergranular M23C6, chromium enriched carbides as well as intragranular TiC precipitates. Complex phases containing Zr, C and S were also observed. Three-dimensional images as provided by the tomographic atom-probe revealed the presence of a strong segregation of both boron and molybdenum at grain boundaries. Considerable chromium enrichment at γ′-γ′ grain boundaries and slight carbon segregation to GBs, whatever their chemical nature, were also detected. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1200 and 800°C. Boron, chromium and molybdenum GB enrichments are thought to be mainly controlled by an equilibrium segregation process. No segregation of zirconium was detected. Copyright \textcopyright 1996 Acta Metallurgica Inc.