Natural gasoline of low octane value derived from natural gas is upgraded to higher octane value by means of a liquid ternary catalyst comprising trifluoromethanesulfonic acid and hydrogen fluoride in conjunction with a Lewis acid of the formula MX.sub.n wherein M is selected from Group III-A, IV-B or V elements of the Periodic Table, x is a halogen and n is a number varying from 3 to 6.