The preparation, characterisation and activity of activated carbon-supported nickel catalysts prepared by impregnation were studied by a range of temperature-programmed techniques including thermogravimetry (TG), differential scanning calorimetry (DSC), temperature-programmed reduction/desorption (TPR/TPD) and temperature-programmed reaction. The impregnation isotherm was interpreted in terms of two nickel phases. The two main peaks observed in the reduction of the precursor by DSC and TPR are consistent with the presence of these two Ni phases. One of them ("interactive" phase), which is bound to the support by interaction with surface functional groups during the impregnation step, is responsible for the high reactivity observed in carbon gasification at low temperatures (LTR). The other ("non-interactive" phase), which is detected in the TPD experiments by the low temperature H-2 peaks, and which results from the salt precipitation in the drying step of catalyst preparation, is proposed to be the main catalyst agent in carbon gasification at high temperatures (HTR).