Thin films, 800-2000 Å thick, of single-crystal (100) sphalerite CdS, which were grown epitaxially on single-crystal (100) NaCl substrates and possessed a density of 1012-13 defects/cm2, were converted to CuxS by dipping them in a hot CuCl solution. Transmission electron microscopy of the converted samples revealed a new CuxS cubic phase of high quality (106-7 defects/cm2), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS to CuxS transformation were carried out by heating CdS films with CuCl layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The new CuxS cubic phase was found to be stable up to 330°C, even in co-existence with CdS.