A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu3 as model systems. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVP-stabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 degreesC, diffusion of Cu into Au occurs, and the atomically disordered solid solution CuxAu1-x exists. For AuCu, nucleation occurs by 200 degreesC, and atomically ordered AuCu exists between 200 and 400 degreesC. For AuCu3, an AuCu intermediate nucleates at 200 degreesC, and further diffusion of Cu into the AuCu intermediate at 300 degreesC nucleates AuCu3. Atomically ordered AuCu and AuCu3 nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 degreesC.