The crystallization of soluble salts is one of the most important factors of wall painting deterioration. These salts are mainly sulphate minerals such as gypsum and can originate from different sources. The master's thesis deals with the baroque wall paintings from the Church of the Annunciation in Ljubljana. In order to determine the type, occurrence and origin of the soluble salts, a total of ten samples of wall paintings from the side chapel of St. Francis were analysed. The type of soluble salts and the occurrence of the salts in the samples were determined by means of powder X-ray diffraction analysis (XRD) and Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS), while the origin of the soluble salts was determined from the isotopic composition of sulphur and oxygen. The results showed that gypsum is present in all samples and that hexahydrite and polyhalite are also present in a smaller proportion. In most samples, sulphatisation of the binder occurred, where gypsum replaced the original lime binder. The sulphatisation is present in some places in the surface part of the sample, where gypsum also forms bands of sulphatisation. Gypsum crystals also appear in the pores and as a coating on the surface of the material. Hexahydrite is also present in the pores of the binder. All samples are enriched in the heavy sulphur and oxygen isotope. The values of the isotopic composition of sulphur and oxygen in the samples are comparable to the values of the isotopic composition of sulphur and oxygen in atmospheric SO2, so the origin of the salts is attributable to atmospheric pollution. A greater variability is detectable in the isotopic composition of oxygen, which may be a consequence of the presence of different sources (SO2 from the atmosphere, CaCO3, CO2, O2 from water etc.).