The development of ternary nanoscale materials with controlled cross-sectional doping is an important step for the use of chemically prepared quantum dots for nanoscale engineering applications. We report cross-sectional, elemental doping with the formation of an alloyed CdSSe nanocrystal from the thermal decomposition of Li-4[Cd10Se4(SPh)(16)]. The sulfur incorporation arises from surface-mediated phenylthiolate degradation on the growing quantum dot surface. In the alloy, we identify a pure CdSe nucleus of similar to 1.5 nm, consistent with the predictions of nucleation theory. As the particle grows, S2- incorporation increases until the CdSSe reaches similar to 4 nm, where a marked reduction in phenylthiolate content on the nanocrystal is observed by CP-MAS NMR spectroscopy, implying that rapid decomposition of the phenylthiolate arises with subsequent enhanced S2- incorporation at the level of the stoichiometry of the reaction (namely similar to 60%). The use of molecular clusters to allow controlled defect ion incorporation can open new pathways to more complex nanomaterials.