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A novel and short method for the synthesis of macrocyclic alkanones has been developed: medium- and large-ring 1-vinylcycloalkanols can be transformed directly and efficiently into the isomeric, ring-expanded bishomologous macrocyclic ketones by a thermal 1,3-C shift rearrangement reaction in the gas phase, carried out under dynamic conditions in a flow reactor system at high temperatures. This two-carbon ring enlargement protocol involves only two steps and is easily repeatable, since the obtained cyclic ketones can directly be used as starting materials for the subsequent analogous two-carbon ring insertion reactions. Alkyl substituents at the vinylic side chain are transferred to the ring-expanded ketone as corresponding ?- and ?-substituents, respectively. Therefore, this thermo-isomerization process provides not only a short route to cycloalkanones but also to many alkyl-substituted macrocyclic ketones, such as, e.g. the musk odorant (±)-muscone. A reaction mechanism via alkyl hydroxyallyl biradical intermediates is proposed.