Aqueous synthesis of mixed cadmium and zinc sulfides colloidal quantum dots (QDs) has been successfully realized. Colloidal CdxZn1−xS QDs are formed in a cubic crystal lattice with particle size of ∼2 nm. The blueshift of optical absorption spectra from 420 to 295 nm and recombination photoluminescence from 646 to 483 nm with increasing zinc content in QDs was observed. Optimum photoluminescence intensity occurs for QDs with Cd0.3Zn0.7S composition. With increasing zinc content up to Cd0.3Zn0.7S, luminescence intensity increases and decreases when zinc content is larger than 0.7. The increase in photoluminescence intensity is explained by the increase in the number of point defects, such as complexes of interstitial metal atoms–metal vacancies [Mei−VMe]. Such complexes occur due to displacement of the metal atom at the center of the elementary tetrahedron due to substitution of one of the four sulfur atoms by an impurity atom, such as an oxygen atom.