The master thesis is defining the problematics of cleaning unvarnished modern oil paintings on two specific paintings, Tulpe by Franc Pavlovec and Brez naslova by Alojzij Kogovšek. Each of the artworks has responded differently on the water-based systems for surface cleaning: the Tulpe’s paint layer is much more sensitive to water compared to the paint layer of Brez naslova. Although the latter is much more stable, it also has more rigid surface impurities. Tendencies of modern painters are presented in order to better understand their choice to use this painting technique. In search of causes for water-sensitivity, the oil paint’s formulation and its production process needed to be understood. Buffer solutions, chelators, emulsions and agar gel are presented along with the pH and conductivity measurements. Hydrophobic coatings, such as cyclododecane and cyclomethicone D5, are offered as an alternative solution. The master thesis also introduces some guidelines by Paolo Cremonesi on how to systematically search for the optimal water-based cleaning system. The original condition of the paintings before restoration as well as the tests results have been described. From both paintings, some samples were taken and analysed in the laboratory (reports enclosed). On the basis of sample research and solubility tests, the causes of water-sensitivity can be presumed. Both paintings have been cleaned of surface impurities and properly restored.