Alloys of composition varying in the system FexNi100-x (x= 25, 50, and 75) with a microstructure constituted in its majority by nanometric particles were successfully produced by thermal decomposition of nitrate solutions followed by reduction with hydrogen at 700 °C.. The composition influenced considerably the equilibrium achieved after the reduction step. The alloy of composition equal to Fe25Ni75 indicated the presence of only one phase, which crystallized in a face centered cubic structure. By changing the composition, the microstructure varied considerably. The alloy of composition Fe50Ni50 had a microstructure characterized by only one simple tetragonal phase. On the other hand, two phases were detected in the equilibrium for the alloy with composition Fe25Ni75, one with body centered structure and the other with a face centered cubic structure. The measurements of saturation magnetization at 300 K as a function of the applied field indicated a superparamegnetic behavior, typical of nanometric particles. Copyright © (2010) by Associação Brasileira de Metalurgia Materiais e Mineração (ABM).