Cuprous oxide (Cu2O) hollow spheres have been successfully prepared via a template-free hydrothermal method with the assistance of poly(vinylprrolidone) (PVP) at 90 degrees C. The structure and morphology of the Cu2O hollow spheres were characterized by X-ray diffraction, field-emission scanning electron microscope, and transmission electron microscope. The as-prepared hollow spheres are cubic phase Cu2O. The as-prepared products have a hollow sphere structure with outer diameters of 400-700 nm and the thickness of shells is about 40-70 nm. Poly(vinylprrolidone) is employed for the first time to improve the stabilization of crystalline phase occurred at a rate commensurate with localized Ostwald ripening and self-transformation for producing Cu2O hollow spheres. A possible PVP-assisted Ostwald ripening process is proposed for the formation of Cu2O hollow spheres on the basis of intermediate products at different growth stages. (C) 2009 Elsevier B.V. All rights reserved.