Hydrogenation of the tricyclic aromatic hydrocarbons acenaphthene and fluorene on industrial aluminum oxide catalysts in a flow system has been studied. It has been found that total these hydrocarbons are exhaustively hydrogenated in the presence of a nickel-chromium catalyst at 200A degrees C and a pressure of 100 atm to give isomer mixtures of the corresponding perhydroaromatic hydrocarbons decahydroacenaphthene and dodecahydrofluorene. The liquid products obtained can be of interest as components of hydrocarbon fuels with increased density. Certain conformational features of the stereoisomers obtained have been considered. It has been assumed that some spatial isomers of decahydroacenaphthene have six-membered rings in the boat conformation.