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The specific heat of Ni(NO, )26NH3 has been measured between 0.4 and 20 K in a mechanical heat switch calorimeter. A sharp X-like anomaly occurs at the Neel temperature T"= 1.345 K. The experimental entropy of the transition was determined to be hS = 9.125 J/Kmol, in quite good agreement with the theoretical value R ln(2S+ 1) = 9, 137 J/Kmol for S = 1. Since EPR measurements indicate that the anisotropy in these crystals is not large, a Heisenberg three-dimensional Hamiltonian has been used for modeling the magnetic behavior.The mean-field approximation indicates that the antiferromagnetic ordering is of the second kind in an fcc lattice. A best fit of the experimental data to a high-temperature expansion yields values for the exchange parameters J, = -0.125 K and J, = -0.38K which are consistent with this kind of order.15 2731