The electronic and optical properties of a series of PAMAM-dendrimer capped CdS quantum dots (QDs) have been investigated with UV-vis absorption (UV-abs), photoluminescence (PL), X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL) techniques. The size-dependent electronic behavior of the QDs was observed in both the UV-abs and XANES (S K-edge) spectroscopy. Findings from the PL and XEOL studies provide complementary information to the UV-and X-ray absorption results and shed further light on the stability of the CdS QDs upon aging.