[en] In this study, the magnetic properties of neptunium tetrabromide were investigated from 4.2 to 20.0⁰K using the 59.54 keV Moessbauer resonance in ²³⁷Np and from 2.2 to 70.5 K using a vibrating sample magnetometer at an applied field of 10 kOe. The magnetic susceptibility data displayed a peak at 12.5 +- 0.5 K characteristic of a transition from a paramagnetic to an antiferromagnetic state. A plot of chi^-1 vs T above 20⁰K revealed the linear behavior of the Curie--Weiss law (chi = C/[T + theta]) with C = 0.834[(emu-K)/mole] and theta = 15.9 K corresponding to an effective moment of 2.58 Bohr magnetons (mu/sub B/) per Np⁴⁺ ion. The Moessbauer spectra exhibited magnetic hyperfine splitting at all temperatures studied with g₀mu/sub n/H/sub eff/ = 10.4 cm/s at 4.2⁰K. A decline of about one percent in H/sub eff/ above 12⁰K along with conspicuous broadening of the spectral lines indicated disappearance of long-range magnetic ordering coupled with paramagnetic relaxation phenomena. Calculations with the Weiss Molecular Field model using the experimental Neel temperature from the susceptibility data predicted a molecular field of the same order of magnitude as the decline observed in H/sub eff/. A relaxation time of 2.5 x 10^-9 s at 20⁰K was given by calculations using the theory of Blume and Tjon modified for ²³⁷Np