Activated carbons are recognised as inexpensive, easily available, and efficient catalysts for tar cracking reactions at high temperatures. Their use in hot syngas cleaning is limited by the rapid deactivation resulting from the coke deposition, and the consequent masking phenomenon over the activated carbon surface. This study contributes to a better understanding of the problem and sheds light on possible solutions, by investigating how the temperature (750 °C-850 °C) and the steam concentration (0%-10%) can affect the extent of the water-gas and reforming reactions occurring between steam and the coke covering the catalyst surface. The activated carbons have been fully characterised before and after the tests utilizing naphthalene as a tar model compound, to study the evolution of their structure after long duration tests. Steam positively affects the naphthalene conversion efficiency, preserving the characteristics of the material. For example, at a temperature of 800 °C and a steam concentration of 7.5%, a naphthalene conversion of over 90% is achieved even after 7 h of testing. XRD and Raman spectroscopy analyses suggest that naphthalene cracking produces a coke layer that is more amorphous and, above all, more reactive towards the water gas reaction than the original activated carbon structure.