We report synthetic routes to both cubic and hexagonal phase Cu_2−x(S_ySe_1−y) alloy nanocrystals exhibiting a well-defined near-infrared valence band plasmon resonance, the spectral position of which is dependent mainly on x, i.e. on Cu stoichiometry, and to a lesser extent on the crystal phase of the NCs. For cubic Cu_2−x(S_ySe_1−y) nanocrystals y could be varied in the 0.4–0.6 range, while for hexagonal nanocrystals y could be varied in the 0.3–0.7 range. Furthermore, the Cu_2−x(S_ySe_1−y) nanocrystals could be transformed into the corresponding Cd-based alloy nanocrystals with comparable S_ySe_1−y stoichiometry, by cation exchange. The crystal phase of the resulting Cd(S_ySe_1−y) nanocrystals was either cubic or hexagonal, depending on the phase of the starting nanocrystals. One sample of cubic Cu_2−x(S_ySe_1−y) nanocrystals, with S_0.5Se_0.5 chalcogenide stoichiometry, was then evaluated as the anode material in Li-ion batteries. The nanocrystals were capable of undergoing lithiation/delithiation via a displacement/conversion reaction (Cu to Li and vice versa) in a partially reversible manner.