Peculiarities of the distribution of high-molecular normal alkylbenzenes in the Vendian-Cambrian oils of the Siberian platform are studied according to the data of chromatomass-spectrometry. The predominance of high-molecular homologues with the odd number of carbon atoms in a molecule is established. The possible biochemical precursors and the probable mechanism of the formation of “odd” monoalkylbenzenes are suggested.