Metal-organic thin films fabricated through industry-feasible atomic/molecular layer deposition (ALD/MLD) routes are highly attractive materials with diverse functional properties, but they suffer from poor chemical stability in ambient (humid) conditions and especially in direct contact with liquids which limits their practical implementation. The most efficient way to protect the inherently unstable thin films is to encapsulate them with chemically inert material layers without exposing the metal-organic material to air during the processing. Here, we demonstrate the robust in situ encapsulation of luminescent ALD/MLD-grown Eu-organic (europium hydroxyquinoline carboxylate) thin films with ultrathin (1-12 nm) ALD-grown Y₂O₃ capping layers deposited under the same deposition conditions. From x-ray reflectivity analysis, the successful capping-layer formation with only a minor etching effect on the underlining Eu-organic film was confirmed despite the use of the strongly oxidizing reactant (O-3) for the ALD Y₂O₃ process. Importantly, the film composition and luminescent properties were not compromised by the etching. The stability of the encapsulated thin films was studied in both dry and humid air, as well as in liquid water. The results revealed that already a 3-4 nm Y₂O3 capping layer effectively increases the Eu-organic film stability both when stored in open air and when exposed to liquid water. The enhanced stability in the liquid environment is, in particular, critical for the use of Eu-organic thin films for bioimaging applications.