CuInS2 nanoparticles (NPs) usually take chalcopyrite-(CP) structure. Recently, CuInS2 NPs with pseudowurtzite (WZ) structure, which is thermodynamically less favored, have been synthesized. However, the formation mechanism of this metastable-phase has not been understood yet. In this report, the key issue of phase selectivity of CuInS2 (CIS) NPs has been investigated using various metal sources and ligands. Experimental results suggested that the crystalline structure and morphology of CIS NPs were decided by the stability of indium ligand complex; the active ligand reduces the precipitation rate of In2S3, resulting in pre-generation of Cu2S seed NPs. Crystallographic analogy and superionic conductivity of Cu2S remind us that the formation of WZ CIS NPs is attributed to the pre-generation of Cu2S seed NPs and the following cation exchange reaction. In order to confirm this hypothesis, Cu2-xS seed NPs with various structures have been annealed in indium-ligand solution. This experiment revealed that the crystalline structure of CIS NP was determined by that of pre-generation Cu2-xS NPs. Our results provide the important information for the phase control and synthesis of ternary chalcogenide NPs with a novel crystalline structure. (C) 2012 Elsevier Inc. All rights reserved.