The physical properties of binary actinides and transition metal alloys are of great importance for the safety assessment of nuclear fuels. Since transition metals are major components of the cladding material of fuel rods (stainless steel, HT-9 etc.), the physical properties of those compounds formed by accidental fuel-cladding interactions could have limiting factors on the fabrication, life time operation and disposal of nuclear fuels. Binary compound of the form ReT5 (Re = rare earth, T = Transition metal) has been in the focus of interest mainly because of their magnetic properties (small Re to T stoichiometric ratio, large spontaneous magnetization and high Curie temperature) and their ability to store large amount of hydrogen per formula unit (f. u.) Reported in 1997 [1], NpNi5 was synthesized and identified to have hexagonal (D2d) CaCu5 crystallographic structure, with room-temperature lattice parameters a = 8.3107(1) Å and c = 8.1058(1) Å. In the present study, NpNi5 has been synthesized and characterized by means of powder x-Ray diffraction (Fig. 1 left panel), Superconducting – Quantum – Interference – Device magnetometry (SQUID, Fig. 1 right panel), 237Np Mössbauer spectroscopy [2] (Fig. 2 left panel) and specific heat measurements (Fig. 2 right panel). Magnetization curves indicate that NpNi5 is a ferromagnet (TC ~ 16 K), fit of the paramagnetic part to the Curie–Weiss law (C~1.7 emu∙K/mol, θP ~ 14.6 K) gives an effective moment μeff ~ 3.7 μB per f.u. – no magnetization hysteresis was observed. The isomer shift (IS  -11.1 mm/s vs. NpAl2) observed in Mössbauer spectra suggests a tetravalent Np state, but considering the influence of conduction electrons we determine a Np3+ (5f4 configuration) oxidation state. The hyperfine field determined by fitting of the spectra (~439T) gives an ordered moment at the Np site μNp ~ 2 μB per Np ion (1 μB = 215 T [3]). The magnetic transition is clearly visible in the temperature dependence of the specific heat, and a magnetic phase diagram as a function of temperature and external magnetic field was generated.