Synthesis of nanoparticles in high-boiling alcohols (so-called polyol synthesis) and surface functionalization of nanoparticles with polyethylene glycol (so-called PEGylation) in combination with certain heating are often accompanied with an intense fluorescence in the blue to green spectral range. Based on the polyol synthesis of Zn-3(PO4)(2) nanoparticles and a critical consideration of the relevant experimental conditions-including the presence of nanoparticles, the role of dissolved metal salts (ZnCl2, MgCl2, KCl), the type of the polyol (DEG, GLY, PEG400), the temperature and time of heating (150-230 degrees C, 1-6 h)-we can correlate the observed fluorescence to the formation of carbon dots (C-dots) stemming from thermal decomposition (i.e., dehydration and carbonization) of the polyol. Thus, the thermal decomposition of polyols results in C-dots with a diameter of 3-5 nm at narrow size distribution. The formation of C-dots is confirmed by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and fluorescence spectra.