The presence of C15+ trans-1-methyl-2-alkylcyclopentanes in crude oil has been proven for the first time by means of the C-13 NMR technique, and a procedure for their identification is described. It has been shown that these hydrocarbons represent a group of compounds bearing structurally different substituents, including the homologous series of trans-1-methyl-2-n-alkylcyclopentanes extended over a broad range of molecular masses. Their concentration in five samples of topped saturated petroleum hydrocarbons varies severalfold, with the maximal value being similar to 1 wt %. The ability of hydrocarbons from this group to form complexes with urea and thiourea was examined. The importance of the analysis of the temperature and concentration dependences of the chemical shifts of signals for the determination of the structure of previously unknown compounds in oil.