Abstract: The mechanism of interaction of adamantane with propylene has been studied by quantum-chemical calculations within the density functional theory (DFT). It has been shown that the main products of adamantane alkylation with propylene in the presence of acid catalysts are hydrocarbons with unbranched (normal-chain) substituents, 1-n-propyl- and 1-n-propenyladamantanes. The main stages of adamantane alkylation and the geometric and electronic structures of the intermediates have been determined. The thermodynamic characteristics of the studied adamantanes have been found, and the mechanisms of individual steps of their transformations have been proposed. The obtained data make it possible to control the process of preparation of substituted adamantanes with a defined structure that are of interest for the development of efficient energy-rich materials, high-density fuels, and thermally stable polymers. © 2020, Pleiades Publishing, Ltd.