Diamondoids were separated from protodiamondoids (precursors of diamondoids) in a saturated fraction (150-350 degrees C) of crude oil by thermal diffusion. It was found that in the course of thermal diffusion separation, C-10-C-13 adamantanes (tricyclo[3,3,1,1(3,7)]decanes) and C-14-C-16 diamantanes (pentacyclo[7,3,1,1(4,12),0(2,7),0(6,11)]tetradecanes) are separated from the corresponding protoadamantanes and protodiamantanes and descend to the lower sections of the thermal diffusion column. In contrast to adamantanes and diamantanes, C-18-C-19 triamantanes (heptacyclo[7,7,1,1(3,15),0(1,12),0(2,7),0(4,13),0(6,11)]octadecanes) are not separated from prototriamantanes and are concentrated in the lower sections of the thermal diffusion column. Catalytic isomerization of this thermal diffusion fraction with aluminum bromide was carried out to further confirm that the numerous compounds that have the same characteristic ions as triamantanes but elute later are polycyclic (most likely bridged) hydrocarbons, specifically prototriamantanes. As a result, all these polycyclic compounds were isomerized into triamantanes. Similarly, in the course of isomerization of thermal diffusion fractions containing protoadamantanes and protodiamantanes, all of these turned into adamantanes and diamantanes, respectively.