The nucleation of CdS quantum dots in a sodium germanate glass matrix by means of isothermal treatment is demonstrated. It is shown that a sufficiently high concentration of cadmium ions in the initial glass results in nucleation of fairly big quantum dots. The luminescence of CdS is mainly of a trap nature and is localized in the region of 600–900 nm. Excess cadmium ions and nucleation of quantum dots in oxide glass lead to the defect nature of the luminescence bands: mainly associated with interstitial cadmium ions (ICd), the donor-acceptor pairs [VCd–VS], and deep trap levels created by volume cadmium vacancies. The luminescence quantum yield of CdS quantum dots is less than 1 %, indicating that nonradiative recombination of charge carriers is the dominant process.