Abstract <p>The methodology and results of studying the main weathering processes of oil spilled in the marine environment are presented in this paper. Evaporation, dissolution, emulsification, and natural dispersion are considered. The influence of oil weathering processes on the selection of oil spill response methods (mechanical, physicochemical, or thermal) was estimated. Tests were carried out using various oil samples recovered in the Russian Federation taking the hydrometeorological conditions of the seas located in temperate latitudes into account. The low significance of natural dispersion on the fate of an oil spill was established (especially under conditions of a calm sea and bituminous superviscous oil). Evaporation and emulsification were confirmed to be the most important oil weathering processes from the point of view of selecting an oil spill response method. The physicochemical properties of oil can change dramatically during these processes. It was shown that the evaporation losses for light low-viscosity oil were much greater than those for bituminous superviscous oil. This was reflected in the density and dynamic viscosity of the resulting residues. It was noted that evaporation of a significant proportion of light oil fractions can negatively affect the effectiveness of dispersant application and in situ burning. A high tendency to be emulsified (regardless of conditions) was revealed for bituminous superviscous oil. This led to obtaining emulsions with very high water contents and dynamic viscosities. The necessity to take the physicochemical properties of residues formed as a result of oil evaporation and emulsification on the water surface into account when selecting an oil spill response method or oil spill modeling method was noted. The compounds (alkanes, cycloalkanes, aromatic hydrocarbons and organic sulfides) that are capable of changing the toxicological characteristics of water were registered in water samples taken after the tests to study the oil dissolution process.</p>