The formation and evolution of carbonaceous deposits on H-mordenite and platinum-doped H-mordenite during conversion of n-butane in a flow reactor were investigated using in situ UV-vis-NIR spectroscopy. When n-butane (10%) was mixed with H-2 (90%). both catalysts exhibited stable performance at a reaction temperature of 623 K and a total pressure of 1 atm, and no unsaturated compounds accumulated on their surfaces. When the H-2 in the feed was replaced with helium, rates jumped to a higher level, but then declined rapidly while unsaturated surface species formed on the zeolite surface. Spectra of both catalysts showed defined bands at 286, 335, 395, and 460 nm; bands in these ranges have been previously assigned to cationic species. Butenes, which were detected in increased concentrations in the absence of H-2 in the feed, are demonstrated to be the precursors of these surface species. Deactivation was more severe for the platinum-doped sample and affected isomerization, disproportionation, and hydrogenolysis functions but not dehydrogenation and H-2 activation. Return to a H-2-containing feed reduced the concentration of unsaturated surface species on the Pt-containing catalyst, with a very slow recovery of isomerization activity. (C) 2012 Elsevier Inc. All rights reserved.