This work studies the heterogeneous conversion of naphthalene over different chars at 800 degrees C and 900 degrees C with gas residence times ranging from 0.05 s to 0.15 s. The aim was to analyse the role of the pore size distribution and surface composition on the catalytic activity of the char during batch experiments with a steady gas flow doped with naphthalene. The char samples were produced by steam activation using the same parent material (a pyrolysed Colombian coal). The activation aimed at generating chars with increasing porosities but similar surface composition. An ash-leached char was also prepared to investigate the role of the ash-forming elements on the char activity. The evolution of the naphthalene conversion in the gas phase and the pore size distribution of the char were measured during the tests. The initial naphthalene conversion and the char deactivation rate were seen to be strongly influenced by the concentration of alkali and alkaline earth metallic (AAEM) elements in the char. In absence of AAEM elements, the char surface was rapidly deactivated due to the depletion of surface oxo-groups produced during the char activation. The steam activation of unleached chars does not influence their inherent surface reactivity but significantly enhances the amount of surface area available for tar conversion. A linear correlation between the reaction rate coefficient and the mesopore surface area was determined for the chars with similar surface composition but different pore size distribution.