Sulfur tolerance and selectivity changes caused by sulfur were investigated on Pt/KL, Pt/SiO2, and Pt-K/SiO2 catalysts for skeletal reactions of n-hexane and dehydrogenation of cyclohexane. Measurement of accessible Pt surface by CO and transmission electron microscopy (TEM) observations were also performed. The results showed that the presence of K+ can be related to both the high aromatization activity/selectivity and the high sensitivity to sulfur poisoning in n-hexane conversion. The decrease in dehydrogenation activity (pure metallic function) caused by sulfur was more pronounced on K+-free catalyst. It is suggested that K+, which stabilizes an intermediate of aromatization, loses this property by its interaction with sulfur. Thus, significant and selective loss of aromatization activity occurs in the presence of sulfur.