Grain refinement to the nanocrystalline scale is known to enhance physical properties as strength and surface hardness. For the case of Au-Cu alloys, development of the pulsed electroplating has led to the functional control of nanocrystalline grain size in the as-deposited condition. The thermal aging of Au-Cu electrodeposits is now investigated to assess the stability of the nanocrystalline grain structure and the difference between two diffusion mechanisms. The mobility of grain boundaries, dominant at low temperatures, leads to coarsening of grain size, whereas at high temperature the process of bulk diffusion dominates. Although the kinetics of bulk diffusion are slow below 500 K at 10(-20) cm(2) s(-1), the kinetics of grain boundary diffusion are faster at 10(-16) cm(2) s(-1). The diffusivity values indicate that the grain boundaries of the as-deposited nanocrystalline Au-Cu are mobile and sensitive to low-temperature anneal treatments affecting the grain size, hence the strength of the material. (c) 2006 Elsevier B.V. All rights reserved.