Using a domestic microwave oven and new, inexpensive precursors, a rapid and reliable synthesis of highly luminescent CdSe/ZnS NPs was developed. To evaluate the quality of our core/shell particles for varying shell thickness in comparison to that of CdSe/ZnS nanoparticles obtained commercially, the parameter fluorescence quantum yield is been used as well as a new, straightforward, thiophenol-based shell-quality test as a tool to ensure a dense ZnS shell without holes and cracks, which is a prerequisite for high luminescence and stability.