<p>Trivalent europium is a well-known phosphorescent probe widely used in lighting devices and displays, bioassay markers, and anti-counterfeiting phosphors. Double halide perovskite nanocrystals (DHP NCs) have emerged in recent years as attractive host lattices for various luminescent lanthanide dopants owing to the diverse range of trivalent substitutional sites. However, the effect of the DHP NC composition on the Eu³⁺ emission properties has not been systematically investigated. In this work, we prepared a series of DHP NCs containing Eu³⁺ by employing a modified colloidal hot injection procedure. The nanocrystals exhibit finely tunable Eu^{3+ 5}D₀−⁷F_J emission peaks, owing to the different dependence of the electric and magnetic dipole f-f transitions on the coordination symmetry of Eu³⁺. A variation in the values of the Eu³⁺ asymmetry factor, defined as ⁷F₂/⁷F₁ peak intensity ratio, between 1.0 and 2.5 was observed for different NC compositions, compared to ca. 5 for Eu³⁺ in an aqueous solution. The asymmetry factor was also found to be dependent on the Eu³⁺ doping concentration for a given DHP NC host lattice. The sensitivity of the relative intensities of Eu³⁺ emission peaks on the DHP NC composition, together with the dual emission of these NCs, is promising for various anti-counterfeiting applications.</p> Graphical Abstract <p></p>