Abstract The catalytic activity of char (fixed carbon + metal oxides) recovered from a commercial waste gasification plant and char-derived ash (metal oxides only) from the oxidation of the char (i.e. the controlled oxidation of the amorphous carbon in the char) was investigated for the cracking and steam reforming of tar. The latter was obtained following the soxhet extraction of sludge collected during a cleaning process (water scrubbing) on primary syngas. The tar was evaporated along with water in an evaporator maintained at 650 °C and entrained by a nitrogen flow. Tar in the gas phase was sent to a reformer where cracking + reforming tests were carried-out at 900 °C and 1 bar using char and char-derived ash pellets as catalysts. These catalysts were characterized using TGA, AE, XRD, SEM, EDX, XRF, ICP-MS, BET and Matersizer. Char-derived ash and char both hold similar metals and mineral structures. Following a 2 h conditioning at 900 °C, the fixed carbon in the char exhibited a larger surface area and pore volume while the char-derived ash revealed larger particle from the merging of vicinal minerals. No metal evaporation was observed after 24 h at 900 °C. During cracking and reforming, the initial 2–6 rings tar mixture extracted from the gasification residues sludge, loaded in the stream of nitrogen + steam at 65 g/Nm3, was reduced to 173.3 and 90.2 mg/Nm3, when using char-derived ash and char respectively. Pure syngas (CO + H2) was produced as the only permanent gases while carbon deposition was noticed in the solid beds. Even though the extent and type of tar were essentially similar when char and char-derived ash were used, the kinetics to reach steady state were slower for the char-derived ash. Steam content did not seem to impact on the tar conversion. However WG and WGS reactions most probably occurred, which might contribute to modulate the H2/CO molar ratio in the produced gas according to its targeted downstream utilization.