The early stages of precipitation of Cr rich precipitates were investigated by Atom Probe Tomography (APT) in a Cu-1Cr-0.1Zr (wt.%). This way, quantitative data were obtained about their size, 3D shape, density, composition and volume fraction. Surprisingly, in the early stage of precipitation, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction with the equilibrium Cu-Cr phase diagram. APT data also show that some impurities (Fe) segregate along Cu/Cr interfaces. A precipitation sequence is proposed and the relationship between mechanical properties and microstructure is discussed.