In this work, some micro-samples belonging to the open-air rock art site of Cueva de la Vieja (Alpera, Albacete, Spain) were analysed. These samples were collected after and before a desalination treatment was carried out, with the aim of removing a whitish layer of concretion that affected the painted panel. The diagnostic study was performed to study the conservation state of the panel, and to then confirm the effectiveness of the treatment. Micro energy dispersive X-ray fluorescence spectrometry, Raman spectroscopy, and X-ray diffraction were employed for the characterization of the degradation product as well as that of the mineral substrate and pigments. The micro-samples analysis demonstrated that the painted layer was settled on a dolomitic limestone with silicon aggregates and aluminosilicates as well as iron oxides. The whitish crust was composed by sulfate compounds such as gypsum (CaSO₄·2H₂O) with a minor amount of epsomite (MgSO₄·7H₂O). An extensive phenomenon of biological activity has been demonstrated since then in almost all of the samples that have been analysed, and the presence of calcium oxalates monohydrate (CaC₂O₄·H₂O) and dehydrate (CaC₂O₄·2H₂O) were found. The presence of both calcium oxalates probably favoured the conservation of the pictographs. In addition, some carotenoids pigments, scytonemin (C₃₆H₂₀N₂O₄), and astaxanthin (C₄₀H₅₂O₄) were characterized both by Raman spectroscopy and by X-ray diffraction. Hematite was found as a pigment voluntarily used for the painting of the panels used in a mixture with hydroxyapatite and amorphous carbon. The results of the analyses of the samples taken after the cleaning treatment confirmed a substantial decrease in sulphate formation on the panel surface.