Studies of methane decomposition were performed over nickel alumina catalysts modified with magnesia. The catalysts were obtained by the co-precipitation method. The properties of catalysts were investigated by the temperature programmed reduction (TPR) and hydrogen desorption (TPD) methods. Methane decomposition was studied by the thermogravimetric and transient temperature programmed reaction methods. The nature of carbon deposits formed under different reaction conditions was investigated by the temperature programmed oxidation method (TPO) and high resolution transmission electron microscopy (HRTEM). An introduction of magnesium to the preparation mixture of catalysts led to the formation of smaller nickel crystallites with stronger adsorption sites. It was found out that the activity of catalysts and the properties of carbon deposits were related to the catalysts composition and the reaction temperature. Carbon filaments with fishbone-like packed graphitic layers were formed at 500 degrees C, while the multi-walled nanotubes were observed after reaction performed at 700 degrees C. The increase of magnesium content led to the increase of the rate of methane decomposition at the initial stages of the reaction. Deactivation of the catalysts was related to the insufficient removal of carbon species from the surface of nickel crystallites and the formation of stable, graphitic carbon deposits covering the surface of metal. (C) 2009 Elsevier B.V. All rights reserved.