The amount and valence state of sulfur in sulfided Pt catalysts was monitored, together with parallel studies of the catalytic properties of these catalysts in order to clarify the catalytic effect of S in different chemical states on reactions of low-temperature n-hexane reforming. Pt black and 6% Pt/SiO2 (EUROPT-1) were sulfided by H-2/H2S. X-ray photoelectron spectroscopy (XPS) detected both sulfate and sulfide. The chemical state of Pt corresponded to metallic Pt with low amounts of S containing surface adlayer and/or PtS present. This could be observed with Pt black and also with EUROPT. With S present, both catalysts lost ca 95% of their activity in n-hexane transformation at 603 K, n-hexenes being the predominant products. Prolonged runs with n-hexane as well as regenerations using O-2 and H-2 removed most sulfidic S from Pt black and about half of the original activity came back. Isomers and methylcyclopentane were produced in this state with high selectivities. Some reactivation of EUROPT (where no sulfate could be detected) occurred within one single run. A uniform scheme is proposed to describe the effects of sulfate and sulfide. Accordingly, sulfate acts as a structural modifier occupying a fraction of the active surface. Sulfide in small amounts, in turn, has a bonding modifier effect and poisons selectively benzene and fragmentation, enhancing (within the reduced activity) the selectivity of Cs-cyclization and, possibly, also that of isomerization.