A new approach to the luminescent thin films deposition of non-volatile and non-soluble lanthanide aromatic carboxylate, based on the formation and decomposition of mixed-ligand complexes, was proposed and exempli-fied for terbium o-phenoxybenzoate thin film deposition. Lanthanide coordination compounds with organic ligands, espe-cially terbium and europium complexes, are of great interest for the use in organic light-emitting diodes (OLEDs) [1–3] because their emission is due to the electronic transitions of the central ions that give a sharp narrow spectral characteristics and in some cases 100% emission quantum efficiency [3]. Three main classes of lanthanide coordination compounds and their derivatives have been under test for potential applications as emitting materials: β-diketonates, pyrazolonates and aromatic carboxylates [1,2,4,5]. However, their performance as electroluminescent materials is still unsatisfactory for their wide practical applications. One of the main disadvantages of the high luminescent β-diketonates and pyrazo-lonates is their poor photo-and chemical stability. Lanthanide aromatic carboxylates, Ln(RCOO) 3 , possess not only good luminescent properties but also good ultraviolet durability and chemical stability [6]. In spite of this, little attention has been paid to lanthanide aromatic carboxylates as prospective materials of emission layers in OLEDs, probably, due to some difficulties with production of their thin amorphous films. Lan-thanide carboxylic polymeric structures [7,8] are poorly soluble and nonvolatile. This prevents the deposition of lanthanide aromatic carbox-ylate thin films by traditional vacuum and solution processing methods. Some chemical approaches have been used to enhance the solubility of terbium aromatic carboxylates and obtain their thin films for OLEDs from the solution [9–11], but they did not completely solve the problem of high quality film deposition. So, adulteration of aromatic carboxylates to organic polymers of transport layers often leads to deterioration of carboxylate photophysical properties [9]. Photophysical properties of terbium aromatic carboxylates also suffer by introduction of long hydro-carbon chain group into the carboxylate ligands for solubility increase [10]. The third way is the transformation of aromatic carboxylates into mixed-ligand complexes (MLCs) with neutral organic donors (L) – Ln (RCOO) 3 (L) n (n= 1, 2) [11]. This approach allows to improve the solubil-ity and in some cases the luminescence efficiency of aromatic carboxyl-ates too, when ligands L take part in the energy transfer processes. However such examples are rare, and besides thin films of MLCs demon-strate lower thermal stability in respect with Ln(RCOO) 3 thin films and thin film contamination hazard due to separate crystallization of neutral ligand L. Here we proposed a new approach to deposition of Ln(RCOO) 3 thin films. The main idea of our approach is based on transformation of in-soluble Ln(RCOO) 3 or Ln(RCOO) 3 (H 2 O) x into soluble Ln(RCOO) 3 (L) n (reaction 1), deposition of Ln(RCOO) 3 (L) n film on substrate from solu-tion and then MLC decomposition by the film annealing, which results in formation of Ln(RCOO) 3 thin film (reaction 2). Ln RCOO ð Þ 3 H 2 O ð Þ x þ L→Ln RCOO ð Þ 3 L ð Þ ð1Þ Ln RCOO ð Þ 3 L ð Þ→Ln RCOO ð Þ 3 þ L↑ ð2Þ To realize this approach it is necessary to find a ligand L that will be able: 1) to form soluble MLCs, 2) to be removed completely from the MLCs by heat treatment, 3) to be volatile enough, not to contami-nate Ln(RCOO) 3 film after decomposition of the MLC.