AbstractControl of morphology and spectroscopic properties during the synthesis of up-converting nanoparticles (NPs) is a great challenge. One of the most popular ways of NPs synthesis is the hydrothermal method, which is relatively simple, effective, environmentally friendly and permits easy control of synthesis parameters. For these reasons, the hydrothermal method was applied for the synthesis of CaF₂:Yb³⁺,Er³⁺ NPs and optimized. The effects of synthesis conditions on the properties of the product were carefully analysed. The tests were performed to check the impact of two surfactants: sodium citrate (NaCit) and ammonium citrate (NH₄Cit), different excess of ammonium fluoride used as a precipitation agent and different volumes of solution with reactants. The type of co-reagent was found to influence the size of the obtained NPs and charge compensation, required after Yb³⁺ and Er³⁺ doping into Ca²⁺ sites. Depending on the synthesis conditions, the formation of Yb³⁺ clusters and alterations in the Yb³⁺ site symmetry were detected. The excitation and emission spectra revealed the importance of the presence of the Na⁺ ions on the energy transfer mechanism and the resulting emission intensity. The presented results show that applying stirring during the synthesis or changing the type of anti-agglomeration agent has a great influence on the luminescence intensity and colour as well as maximum of excitation when Yb³⁺ ions are used. Analysis of the excitation spectra and Yb³⁺ emission decays showed the complex structure of CaF₂:Yb³⁺,Er³⁺ NPs, with Yb³⁺ ions in two different environments within the volume of NPs with different site symmetries. The samples prepared in the presence of Na⁺ ions were characterized by long Yb³⁺ emission rise times, revealing energy migration between Yb³⁺ at different symmetries and, at the same time, improved the overall luminescence intensity of NPs.