Inter-diffusion at the Cu(In,Ga)Se-2 (CIGS)/CdS interface of high-efficiency CIGS thin film solar cells has been investigated using energy dispersive X-ray spectroscopy (EDX) and field emission transmission electron microscopy (FETEM). CdS thin layers were epitaxially grown by chemical bath deposition (CBD) at a maximum bath temperature of 80 degrees C on CIGS thin films grown with a molecular beam epitaxy (MBE) system. EDX analysis revealed that Cd was present in the thin CIGS layer approximately 100 Angstrom from the interface boundary. In contrast to the diffusion of Cd. the Cu concentration decreased near the surface: of the CIGS film, suggesting substitution of Cd for Cu atoms. These results are direct evidence of Cd-doping into CIGS thin films during the CBD process. On the other hand, no Cd was detected at the CIS single crystal/CdS interface within the detection limit of EDX. This is presumably because of a lack of Cu deficient surface layer in which Cd-diffusion easily occurs. Details of the CIGS/CdS interfacial nano-structure, which may play a role in diffusion propel tics are also presented.