Determination of tetramantanes in crude oils is complicated by low concentrations, the lack of information on retention times, and the fact that their characteristic m/z 292 and 291 mass chromatograms typically contain a variety of foreign peaks having the same ions. Since tetramantanes could not be isolated in pure form using separation by thermal diffusion, the catalytic isomerization of the thermal diffusion concentrate of polycyclic hydrocarbons (most likely, prototetramantanes) over aluminosilicate has been carried out; as a result, all the three tetramantane isomers (iso-, anti-, and skew-) and methyltetramantane have been obtained in pure form. Mass chromatograms at m/z 291 and 292 before and after the separation by thermal diffusion and after the isomerization of the concentrate of saturated polycyclic hydrocarbons on aluminosilicate are presented. The retention indices of tetramantanes on HP-5 are as follows: iso-tetramantane (2293), anti-tetramantane (2365), and skew-tetramantane (2431). It is interesting that methyltetramantane, a homologue with a higher molecular weight, elutes before iso-tetramantane, the tetramantane isomer with the lowest boiling point. Its retention index on HP-5 is 2287.