The gas-phase dehydrocyclization of diphenylamine to carbazole was studied in a fix-bed reactor over Pt and I'd catalysts. Alumina, magnesium oxide and silica were used as supports. The reaction was carried out at a temperature of 560 degrees C in the presence of hydrogen. Pd catalysts are reasonably active but not selective compared to Pt catalysts which possess very high activity and selectivity. For supporting active metal, proper alumina has advantages compared to silica and magnesium oxide. This is due to its acid sites which accelerate the isomerization step. Though higher loading of Pt has a positive influence on the carbazole formation, at a Pt content higher than 0.6 wt.% this effect is negligible. In the presence of catalyst with 0.6 wt.% Pt deposited on alumina, carbazole is produced with a selectivity of about 90% at a conversion of diphenylamine about 97%. (c) 2007 Elsevier B.V All rights reserved.