We have demonstrated a soft solution-processing route for selective synthesis of various phases of nickel sulfides, in which a novel liquid-solid interfacial reaction between a nickel substrate and sulfur is involved, and the reaction of the Ni2+ ion with sulfur in different solvents at low temperatures (less than or equal to200degreesC). Nanostructured Ni3S2 thin films. NiS (millerite) nanowhiskers, and NiS2 single crystals with dodecahedron {101} unit polytype shape were directly synthesized in ethylenediamine. Similar reactions in other solvents were also investigated. In addition, a new molecular precursor Ni(en)(x)-S-y (where (en) is the ethylenediamine ligand) with the composition Ni(en)(3)S-4 derived in ethylenediamine was found to be an excellent precursor for synthesis of nickel sulfide nanoparticles. The reaction mechanism, phase formation, and transformation process are proposed. Nickel sulfide materials with different phases and special shapes could find applications in catalysis, semiconductors, and magnetic devices.