This study provides a detailed characterisation of a leaf protein concentrate (LPC) extracted from Cichorium endivia leaves using a pilot scale process. This concentrate contains 74.1% protein and is mainly composed of Ribulose-1,5-BISphosphate Carboxylase/Oxygenase (RuBisCO). We show that the experimentally determined extinction coefficient (around 5.0 cm−1 g−1 L depending on the pH) and refractive index increment (between 0.27 and 0.39 mL g−1) are higher than the predicted ones (about 1.6 cm−1 g−1 L and 0.19 mL g−1, respectively). In addition, the UV–visible absorption spectra show a maximum at 258 nm. These data suggest the presence of non-protein UV-absorbing species. Chromatographic separation of the concentrate components in denaturing conditions suggests that RuBisCO SC may be covalently bounded to few phenolic compounds. Besides, the solubility of LPC proteins is higher than 90% above pH 6. Such high solubility could make LPC a good candidate as a functional food ingredient.