This article describes the synthesis of carbon nanotubes (CNTs) and carbon nanotibers (CNFs) by the catalytic decomposition of acetylene at 973 K over several Fe/silica catalysts (sol-gel method prepared) in a fluidized bed reactor. Characterization of the catalysts and the products was performed by chemical analyses, N-2 adsorption isotherms (BET surface area), temperature-programmed reduction (TPR), CO volumetric chemisorption, X-ray diffraction (XRD), temperature-programmed oxidation (TPO), and transmission electron microscopy (TEM). An apparent relationship was found to exist between the metallic iron content of the catalysts, metal particle size distribution on the surface of the silica support, and final characteristics of the carbon products obtained. High-purity CNTs and CNFs were achieved after acid treatment of the catalytically produced carbon deposits. Difficulties in exact quantitative characterization of pure CNTs in the presence of other carbon species (CNFs, capsules of carbon, amorphous carbon, etc.) are also reported and discussed. (C) 2003 Elsevier Science (USA). All rights reserved.