The formation and catalytic effect of carbonaceous deposits was studied on monofunctional Pt catalysts: Pt black (PtN, i.e., reduced with hydrazine), Pt/SiO2 (EUROPT-1). Pt on "herringbone" graphite nanotiber (Pt/GNF-H, GNF being able to store hydrogen) and Pt/CeO2 (ceria tending to consume spilt over hydrogen). They were exposed to hexane or t,t-hexa-2,4-diene between 483 and 663 K, with or without H-2. Hydrocarbon transformations during these deactivating exposures as well as in subsequent standard test reaction with hexane in hydrogen excess were studied. Carbon accumulation on Pt black after analogous deactivating treatments was also examined by electron spectroscopy (XPS and UPS). The abundance of hydrogen on Pt sites controlled the activity and selectivity containing much Pt-C species. The amount of surface C could reach similar to 45% causing almost total activity loss, but even similar to 30% C on Pt blacks decreased markedly the catalytic activity, due to massive 3D deposits. "Disordered" carbon selectively poisoned the formation of saturated C-6 products and fragmentation. The yield of dehydrogenation to hexenes was a good universal indicator of deactivation for each catalyst. Four regions were distinguished: "beneficial", "selective", "non-selective" and "severe" deactivation. (c) 2004 Elsevier B.V. All rights reserved.