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Lanthanide upconversion luminescence in nanoparticles has prompted continuous breakthroughs in information storage, temperature sensing, and biomedical applications, among others. Achieving upconversion luminescence at the molecular scale is still a critical challenge in modern chemistry. In this work, we explored the upconversion luminescence of solution dispersions of co-crystals composed of discrete mononuclear Yb(DBM)₃ Bpy and Eu(DBM)₃ Bpy complexes (DBM: dibenzoylmethane, Bpy: 2,2'-bipyridine). The 613 nm emission of Eu³⁺ was observed under excitation of Yb³⁺ at 980 nm. From the series of molecular assemblies studied, the most intense luminescence was obtained for a 1 : 1 molar ratio of Yb³⁺ : Eu³⁺ , resulting in a high quantum yield of 0.67 % at 2.1 W cm^-2 . The structure and energy transfer mechanism of the assemblies were fully characterized. This is the first example of an Eu³⁺ -based upconverting system composed of two discrete mononuclear lanthanide complexes present as co-crystals in non-deuterated solution.