The role of the solvent in the preparation of ternary CuInX2 (X = S or Se) nanocrystals was investigated. It was found that the use of oleylamine as a solvent with copper(II) oleate, indium(III) acetate diphenyl diselenide as precursors yields copper-rich, wurtzite-type Cu2.0In1.0Se2.5 (2Cu(2)Se-In2Se3) nanocrystals which after size sorting exhibit the quantum confinement effect. Changing oleylamine for 1-octadecene, while keeping the same set of precursors and reaction conditions, results in the formation of indium-rich chalcopyrite nanocrystals Cu1.0In1.8Se3.2 (Cu2Se-1.8 In2Se3). The differences in the stoichiometry can be rationalized on the basis of stronger reducing properties of oleylamine which more effectively reduces Cu(II) to Cu(I) in the reaction medium. Stoichiometric chalcopyrite CuInSe2 nanocrystals can be obtained in the same conditions by exchanging diphenyl diselenide for the Woollins' reagent, never previously applied to the preparation of Cu In Se nanocrystals. In the case of Cu In S nanocrystals the chemical composition is governed not by the type of the solvent but by the reducing properties of 1-dodecanethiol, the precursor of sulfur. Independently of the solvent copper-rich nanocrystals are obtained of either wurtzite-type Cu1.6In1.0S2.3 (1.6 Cu2S-In2S3) in 1-octadecene or chalcopyrite-type Cu1.8In1.0S2.4 (1.8 Cu2S-In2S3) in oleylamine. (C) 2015 Elsevier B.V. All rights reserved.