TEM and atom-probe techniques are used in a combined way for the study of straight and serrated grain boundaries in nickel base superalloys such as Astroloy. These two types of boundaries lead to very different mechanical properties. Experimental conditions are discussed. It is shown that the investigation of boundaries which are perpendicular to the tip axis may lead to biased composition data. Concerning serrated boundaries, microanalyses show that boron and a small amount of carbon segregate at grain boundaries, however, zirconium does not. \textcopyright 1993.