In the thesis we discuss changes in viscosity of the oil spills in the marine environment. After the spill the following processes occur: evaporation, emulsification, mechanical spreading, dissolution, dispersion, oxidation, sedimentation and biological degradation. Evaporation, emulsification and the environmental factors, the change in temperature and the wind, have the greatest influence in Mackay's methods for calculating the viscosity of the spilled oil. With the three Mackay's empirical equations we can determine the oil viscosity at a given time after the spill from different input data. Evaporation is calculated according to the Fingas method, based on laboratory experiments and empirical equations wherein dependency of the evaporation from temperature and the time is given. With the newly developed model VISKOZNOST we can simply and quickly calculate the viscosity of several different oil types in given environmental conditions. The application of the model is presented on two types of oil: Brent Blend and Maui. The oil viscosity is an important parameter in oil spill studies and the developed model can be used as a useful tool in the framework of the teaching process.