Epitaxial films of cuprous oxide, Cu2O, are electrodeposited from aqueous solution onto single-crystalline Si(001). The epitaxial growth of Cu2O onto Si is an unexpected result, because there is a strong driving force for the oxidation of Si to form a native amorphous SiO2 layer. The Cu2O films are single-crystalline in nature, with little or no fiber texture. High-resolution X-ray diffraction shows that the very large lattice mismatch of -21.4% is reduced to +11.2% by the formation of a Cu2O(001)[100]HSi(001)[110] orientation relationship, in which the film is rotated 45degrees around the common [001] axis. Cross-sectional transmission electron microscopy studies suggest that the growth mechanism involves an initial electroless deposition Of Cu2O epitaxial seeds in direct contact with the Si(001), with the concomitant oxidation of Si to form a layer of amorphous SiO2 about 3 nm thick.. Electrochemical deposition Of Cu2O then proceeds on the epitaxial seeds, with the deposit growing not only perpendicular to the surface, but also laterally across the SiO2 interlayer. This type of lateral overgrowth is used in the production of semiconductor-on-insulator (SOI) structures to avoid defects due to misfit strains.