Small area CdTe/CdS solar cells were fabricated using chemical bath deposited CdS and CSS deposited CdTe thin films to investigate the interface properties related to the CdS processing. The effect of post deposition annealing of CdS on the junction properties and the possible interdiffusion at the interface is discussed. The hypothesis of hardening of CdS due to annealing against the diffusion of "S" and "Te" is discussed using the quantum efficiency data in the blue and red regions. Devices prepared using as-deposited CdS films exhibited evidences of higher "S" and "Te" diffusion compared to devices made using CdS films annealed in oxygen. The maximum efficiency of the devices used in this study was 9.8%. (C) 2011 Elsevier Ltd. All rights reserved.