Monometallic Fe, Co, and Ni/MgO catalysts with 50 wt.% metal loadings were prepared and examined for natural gas decomposition to nanocarbonaceous materials, particularly multiwalled carbon nanotubes (MWCNTs) and co-valuable hydrogen. The catalytic testing was carried out in a fixed-bed horizontal reactor at 700 degrees C under atmospheric pressure. The fresh and/or used catalysts were characterized using XRD, TPR, HRTEM, SEM, TG/DTA, Raman spectroscopy, and BET surface measurements. The resulting data showed that the 50%Co/MgO catalyst displayed higher catalytic decomposition activity of natural gas to COx- free hydrogen production (similar to 88%), higher yield of MWCNTs, and excellent stability up to 10 h time-on-stream. On the other hand, the Ni-containing catalyst showed lower catalytic activity toward hydrogen and CNTs production, principally due to the formation of rock-salt MgxNi(1-x)O solid solution as observed from XRD and TPR data. Accordingly, the concentration of Ni particles required for natural gas feed was extremely low. The d orbital of Ni was presumed to be occupied during the formation of the solid solution, which inhibits the solublization or adsorption of hydrocarbons on Ni particles. The MWCNTs obtained over Ni-based catalyst had narrow and homogeneous diameters (similar to 11-13 nm). However, the Fe/MgO catalyst exhibited intermediate activity between those of Ni and Co=MgO catalysts toward hydrogen production (similar to 44%). This catalyst produced mixtures of carbon nanofibers and nanotubes.