Molecular-based magnet nickel-iron cyanide, K0.8Ni1.1[Fe(CN)(6)]. 4.5H(2)O, was prepared by coprecipitation. The powder X-ray diffraction indicates an fee crystal structure with a unit cell constant of 10.4 Angstrom. Its Curie temperature was determined to be 28 K from the Curie-Weiss law. The effects of heat treatment on the microstructures as well as magnetic properties in the compound were studied. Several structures were encountered during the course of annealing from room temperature to 600 degrees C. The conversion of the ferromagnetic Fe-III-CN-Ni-II linkage to a paramagnetic Fe-II-CN-Ni-III linkage occurred after heat treatment at 150 degrees C in argon. There was another cyanide structure with a tentative composition of (Ni, Fe)(CN)(2-3)O1-2, which possessed a Curie temperature of 50-60 K and is expected to be ferrimagnetic. Above 550 degrees C, the cyanide compound decomposed into a mixture of fee Fe-Ni and amorphous carbon. In a separate experiment, fairly pure fee Fe-Ni particles were prepared by microemulsion method followed by decomposition at 600 degrees C under a partial pressure of air. The ultrafine particles were found to have a mean particle size of similar to 10 nm, a saturation magnetization of 110 emu/g, and a coercivity of 0.3 kOe.