Formation and reactivity of carbon in Ni/Al2O3 catalysts of methane dry reforming (MDR) have been studied at their interaction with CH4, CO2, H-2 or CH4-CO2 mixture using thermogravimetry and mass-spectrometry. Temperature ranges of carbon accumulation and removal have been established, with a temperature hysteresis of carbon accumulation being observed in CH4-CO2 mixture. The primary deposited carbon blocks MDR activity, but the deactivation is reversible under MDR conditions. The primary carbon transforms into the secondary nanofiber carbon which is stable under MDR conditions and less blocks MDR activity. Effect of Ni catalysts deactivation because of carbonization can be reduced, if at the initial stage the catalyst does not contact with the reaction mixture at temperatures below 700 degrees C. The rate of carbon accumulation was increased with increasing Ni particle size from 2 to 5 nm. (C) 2012 Elsevier B.V. All rights reserved.