A new organometallic "cold-slow" route to strongly fluorescing CdTe/CdS (core-shell) colloids and transparent films is presented. Based on the optical absorption, fluorescence, FTIR, micro-Raman, XPS, and XRD data collected on these nanostructures before and after thermal annealing, a mechanistic path of the core-shell formation and thermal break up is proposed and discussed. The processing of the CdTe/CdS nanostructures starts with 0.5 M tributylphosphine (TBP) stabilized CdS colloid in dichloromethane as a solvent. This yellow colloidal oil composed of 3-4 nm CdS clusters is reacted with liquid Bis(trimethylsilyl)-telluride (TMS2Te) in the presence of excess insoluble CdCl2 salt. During this reaction, a rapid chalcogen atom exchange occurs within a few seconds which produces a new CdTe "core". The expelled sulfide reacts slowly with the CdCl2 salt to form new CdS clusters after several hours. Furthermore, this "CdS-formation-driven CdCl2 salt dissolution" activates a strong green-yellow fluorescence indicating a possible evolution of a "core-shell"-like CdTe/CdS structure. Thermal sintering of the subsequently prepared CdTe/CdS films between 100 and 200 degrees C completely suppresses the fluorescence and initiates CdTe cluster growth, reflecting a high thermal sensitivity of the "core-shell" interfaces. By further raising the sintering temperature to 300-400 degrees C, the TBP ligands are released and, consequently, bare CdS- and CdTe nanocrystals, as well as ternary nanocrystalline CdTexS1-x phases, start forming. Above 400 degrees C, the CdTe part of the nanostructures sublimates, yielding (111)-oriented CdTe films.