During isothermal aging of Cu-Sn films, in situ stress measurements showed tensile stress evolutions in the early stage and compressive stresses finally. The tensile force resulted from the volume shrinkage by reactive interdiffusion, while the compressive stress resulted from the volume expansion by the dominant diffusion of Cu into Sn. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.