The solid-state reaction in an Al/Pd lateral diffusion couple starts with the growth of the crystalline Al3Pd2() phase. When the growth of Al3Pd2 reaches its critical length, the quasicrystalline decagonal Al3Pd() phase starts to grow and both the Al3Pd2 and Al3Pd phases continue to grow afterwards. We have fitted this simultaneous growth with a recently proposed model of kinetic suppression of the second phase. The kinetic analysis directly links the appearance of the decagonal phase as the second phase to its interfacial reaction barrier. We also suggest that the growth of the metastable decagonal Al3Pd phase over the equilibrium phase is a result of its relatively lower interfacial reaction barrier.