The characteristics of carbon nanofibers generated during ethylene decomposition over supported nickel can be readily manipulated by a judicious choice of the support material. The occurrence and ramifications of Ni/support interaction(s), in terms of Ni particle size/morphology/orientation, are considered and related to the carbon structure/dimensions and yield. A 7 +/- 1% w/w Ni loading was achieved by standard impregnation of SiO2, Al2O3 MgO, Ta2O5, activated carbon (AC), and graphite: the reduced catalysts have been characterized by H-2 chemisorption, CO chemisorption/temperature-programmed desorption (TPD), and high-resolution transmission electron microscopy (HRTEM). The reaction of ethylene with hydrogen over these catalysts also generated ethane via hydrogenation, a step that was favored over Ni/Al2O3 and Ni/MgO. Carbon yield (where T < 800 K) increased in the sequence Ni/Al2O3 - Ni/MgO < Ni/AC < Ni/graphite < Ni/Ta2O5 < NiSO2; at higher temperatures (> 850 K), Ni/AC and Ni/graphite delivered the highest yields. With the exception of Ni/graphite and Ni/Ta2O5, which produced helical and highly curved fibers, the other supported Ni catalysts generated a relatively straight (limited curvature) fibrous growth. The occurrence of Ni fragmentation and secondary fiber growth from such fragments is illustrated and discussed. The influence of H-2 content in the feed was investigated with respect to both carbon yield and structure; an increased H-2 content served to enhance fiber structural order. Temperature-programmed oxidation studies have been used to probe the graphitic nature of the carbon product; the results are consistent with HRTEM analysis. © 2003 Elsevier Inc. All rights reserved.