Guinier-Preston (GP) zones in Al-3 at.% Ag were studied by high-resolution Z-contrast scanning transmission electron microscopy. The favorable atomic number (Z)-contrast of Ag atoms dissolved in Al and the limited delocalization of the object information by the imaging process allow nanometer-sized Ag-rich pre-precipitates to be analyzed, even on an atomic level. GP zones with a diameter of about 3 nm in Al-3 at.% Ag aged for 15 h at 105 degreesC, for 4 h at 140 degreesC and for 3 min at 180 degreesC reveal a common structural feature. The zones consist of a Ag-rich shell and a Ag-depleted core. This result confirms previous conclusions based on small-angle X-ray scattering and neutron scattering. (C) 2002 Elsevier Science B.V. All rights reserved.