The present invention relates to a novel thermo-isomerization method for rapidly and simply producing macrocyclic ketones of the formula Ia or Ib. ##STR1## The macrocyclic ketones are prepared in the gas phase at temperatures above 500 C. rapidly and in a simple manner from alcohols of the formula IIa and IIb directly with a high yield. ##STR2## In the formulae Ia, Ib, IIa and IIb, R1-R5, and n have the meanings given in the description.