We present a study on the formation mechanism of copper zinc tin sulphide (CZTS) nanoparticles prepared from metal salts and 1-dodecanethiol in oleylamine. The nanoparticle sample exhibits two kinds of crystals, large ones with a size of about 70 nm and smaller ones with a diameter of about 25 nm. Analysis of the chemical composition of single nanoparticles revealed that the small nanoparticles consist of copper sulphide, while the larger ones are CZTS nanoparticles. A thorough investigation of the formation process of the nanoparticles revealed that the presence of the copper sulphide nanoparticles originates from the formation of copper sulphide nanopartides in the initial nucleation step. These initial copper sulphide nanoparticles are converted to copper tin sulphide before the final CZTS nanoparticles are obtained. (C) 2014 Elsevier B.V. All rights reserved.