CdTe-based quantum dots (QDs) have been used as active materials in various applications, such as sensing, imaging, and light-harvesting devices where the QDs are continuously illuminated by suitable lights. Passivation of CdTe QDs with stable inorganic layers to form core/shell or core/multiple-shell structures has been demonstrated to improve the stabilities of QDs against illumination. However, information related to the UV stability of newly developed CdZnTeS alloyed QDs has not been fully explored yet. Herein, we synthesized CdZnTeS alloyed QDs of different sizes and compared their size-dependent stabilities under UV irradiation. Upon UV exposure the first excitonic peak of QDs blue-shifted, gradually and the relative Cd concentration decreased indicating that QDs were steadily dissolved. We found that the smaller QDs dissolved faster than the larger QDs. By correlating this with the change in the QDs' crystalline structure corroborated by X-ray diffraction studies, we demonstrated that the alloyed structure with a sulfide-rich surface enhances the stabilities of larger QDs. The size-dependent stabilities of alloyed QDs demonstrated here provide information for selecting the right QDs for specific applications.