Hollow and concave nanocrystals find applications in many fields, and their fabrication can follow different possible mechanisms. We report a new route to these nanostructures that exploits the oxidation of Cu2-xSe/Cu2-xS core/shell nanocrystals with various etchants. Even though the Cu2-xSe core is encased in a thick Cu2-xS shell, the initial effect of oxidation is the creation of a void in the core. This is rationalized in terms of diffusion of Cu+ ions and electrons from the core to the shell (and from there to the solution). Differently from the classical Kirkendall effect, which entails an imbalance between in-diffusion and out-diffusion of two different species across an interface, the present mechanism can be considered as a limiting case of such effect and is triggered by the stronger tendency of Cu2-xSe over Cu2-xS toward oxidation and by fast Cu diffusion in copper chalcogenides. As the oxidation progresses, expansion of the inner void erodes the entire Cu2-xSe core, accompanied by etching and partial collapse of the shell, yielding Cu2-xSySe1-y concave particles.