Double labeled 1,4-C-13 butane, (CH3)-C-13-(CH2)-C-12-(CH2)-C-12-(CH3)-C-13 was used to test four industrial isomerization catalysts based on chlorinated alumina. Mass-spectrometric analysis of the iso-butane product shows that extensive intermolecular scrambling occurs in the conversion from n- to iso-butane. Apparently, the reaction mechanism over these catalysts involves a bimolecular reaction, leading to an adsorbed C-8(+) intermediate. At low conversion, the fragmentation pattern of n-butane reveals an intramolecular rearrangement of carbon atoms, i.e. place exchange of C atoms in methyl and methylene groups. Both phenomena had been observed previously for sulfated zirconia (SZ) catalysts; however, for the present catalysts the distribution of the five isotopic iso-butane molecules does not obey the binomial law. The results suggest that isomerization of the C-8 intermediate is less extensive than over sulfated zirconia; beta-fission of the 2,4,4-trimethylpentyl ion leads to two fragments with iso-butyl structure with (2+n) and (2-n) C-13 atoms, respectively.