Ni-based catalysts are very effective and largely used for the steam reforming of heavy hydrocarbons (tars) generated from biomass gasification, even if Ni suffers from deactivation issues mainly due to carbon deposition, sintering and sulphur poisoning. Mayenite (Ca12Al14O33), as support for Ni, has been previously studied and has shown excellent oxidation properties that increase the resistance of the catalyst to carbon deposits. Nevertheless Ni/Mayenite catalyst still suffers from sulphur poisoning. Ceria (CeO2) is well known to be an effective promoter that can bring several advantages to catalysts, such as enhancing the resistance to carbon deposits and increasing the sulphur tolerance of nickel. Here CeO2 is added to Ni/Mayenite catalysts to evaluate its promoting features and resistance to deactivation. In this study Ni/Mayenite and Ce/Ni/Mayenite catalysts were synthesized, characterized and tested in the steam reforming of tar model compounds. The catalytic tests were carried out under different operating parameters (temperature, space velocity and nature of tar compounds) and in the presence of sulphur, and the used samples were characterized to identify the reasons of deactivation.