Conversion of n-butane was studied between 523 K and 623 K over acidic mordenites with SiO2/Al2O3 ratios between 10 and 20. The main products were iso-butane, propane, and pentane. The selectivity to olefins, methane, ethane, C-6 paraffins, and aromatics was lower than 4 mol% under all reaction conditions. Isomerization and disproportionation were identified to be the dominating reactions. At 523 K, both reactions are suggested to proceed via dimeric intermediates. Their relative contributions to the total conversion depend on temperature, partial pressure, and the concentration of the acid sites of the catalyst. The highest yield in iso-butane was achieved over a catalyst with a low density of acid sites and at low temperature. At higher temperatures, the rate of isomerization decreased in favor of disproportionation and cracking. A mechanistic model for the isomerization is proposed.