The application of a range of experimental techniques shows that "amorphous nickel sulfide" (the material precipitated from aqueous solutions of Ni-II salts and SII- under ambient conditions) is actually a hydrated nanoparticulate material with an approximate formula NiS center dot 1.5H(2)O. The particles comprise a crystalline, anhydrous core (diameter ca. 1-3 nm) with the millerite (NiS) structure, surrounded by a hydrated shell phase. The materials prepared under acidic conditions (pH = 3 and 5) transform with age to form polydymite (Ni3S4) and heazlewoodite (Ni3S2), while materials prepared at pH = 7 and 9 do not undergo this transformation. At pH = 12, the preparation procedure yields NiAs-type NiS as a metastable phase.