Al-Ni-based binary alloys offer several intermetallic phases of immense technological importance. We have attempted to understand the growth of interface alloy in an Al-Ni multilayer sample as a function of annealing time using primarily x-ray reflectometry (XRR) and polarized neutron reflectometry (PNR). Powder x-ray diffraction was also used to determine various crystallographic phases in the sample. The multilayer showed remarkable stability with respect to annealing time, following an initial alloy formation at the interface. Stability of such multilayers is important for their applicability as corrosion resistant coatings as well as metallization layers in microelectronic devices. Using XRR and PNR data we have identified the interface layer as Al₃Ni intermetallic phase. Magnetic depth profile obtained from PNR shows that the interface alloy layer is magnetically dead. From the Bragg peak intensities of polarized neutron reflectivity measurements, we have estimated the diffusion lengths after annealing at 160 °C for 1-8 h.