Ni-CeO2, Ni/Co-CeO2, and Ni-Ca12Al14O33 were synthesized by the auto-combustion method and tested as catalysts in the steam reforming of tar model compounds in a fixed bed reactor. Toluene, phenol, and n-heptane were chosen as representative of the different classes of organic compounds that can be found in tar. The catalysts were characterized by X-ray diffraction (XRD) and temperature-programmed reduction (TPR). From XRD analysis it was observed in all the synthesized catalysts the presence of two phases, NiO and CeO2 or Ca12Al14O33. A stronger interaction of NiO with mayenite structure, compared to that of NiO with CeO2, was also shown by TPR analysis. The best performances in terms of conversion and stability were obtained when Ni supported on mayenite was used, confirming the higher redox properties of this support that confers to the catalyst a better resistance to deactivation by carbon deposition. The lower performances observed for Ni supported on CeO2 in terms of conversion and activity were substantially improved by partial substitution of Ni with Co, confirming its ability to increase the Ni catalytic activity and to enhance the reforming of oxygenated species. The apparent kinetic parameters calculated for all the catalysts and the model compounds confirm the obtained experimental results.