Using model catalytic reactions of n-hexane conversion and the conversions of both narrow and wide fractions of actual hydrocarbon mixtures, straight-run gasoline and gas condensate, as an example, it has been shown that modified type-III shungite rock (carbon content of 20-35%) from the Zazhogino deposit of Karelia at certain temperatures exhibited catalytic activity in the isomerization reaction of hydrocarbons with or without the insignificant formation of environmentally hazardous aromatic compounds. In the case of n-hexane and, especially, straight-run gasoline, the octane rating of the product increases.