Diffusion-welding of 304L stainless steel with Zircaloy4 was performed with a copper interlayer. All the samples are composed of a reaction layer and a diffusion-affected zone in the Zircaloy4. Two reaction regimes were identified: (i) involving a liquid phase on the Zircaloy4 side leading to a fast dissolution and (ii) involving solid state reaction on the 304L stainless steel which is consequently less affected. The reaction layers contains numerous phases that were identified as the C36- and C14-Fe2Zr Laves phases and the cubic and tetragonal polymorphs of the FeZr2 intermetallic. The multiplication of the phases with Cu addition results in the interaction with chromium since the Cr-rich C36 phase and the t-FeZr2 phases have limited solubility in Cu and Cr, respectively, leading to the stabilization of phases where this solubility is increased, e.g. the C14 Laves phase and the c-FeZr2. While in the processing conditions tested the copper interlayer was totally dissolved for all the temperature >1223 K, leading to the occurrence of embrittling Laves and intermetallic phases, the presence of copper lead to a significant softening of the weld. The best properties were obtained with the microstructures formed at 1223 K.