The solid-state reaction between Al and Pd is studied in a lateral Al–Pd diffusion couple in the temperature range between 250°C and 430°C. The first reaction phase is the crystalline Al3Pd2phase. After this phase reaches a critical length of around 6 μm, the quasicrystalline decagonal Al3Pd phase starts to grow in between the Al and the Al3Pd2phases. After the appearance of the decagonal phase, both the crystalline Al3Pd2and the quasicrystalline Al3Pd phase continue to grow.