In this work we investigate the effects of a CdCl2 heat treatment on the interface of CdTe/CdS heterojunction solar cells using Rietveld analysis of X-ray diffraction patterns. Although the Rietveld method is an important tool for the study of materials by X-ray diffraction, there have been few reports of its use in thin film analysis. The results showed the occurrence of interdiffusion in the CdS-CdTe boundary, with substitution of tellurium by sulfur in the CdTe lattice that resulted in a CdSxTe1-x alloy. The sulfur content of the alloy was greater than in previous studies. This is attributed to the low oxygen concentration during processing and the strong (111) texture of the CdTe films. (C) 2013 Elsevier B.V. All rights reserved.