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Abstract: The activation of a spent tyre pyrolysis char using CO2 and steam was experimentally investigated, focusing on the pore development of the char during activation. The pyrolysis char, produced in an industrial scale retort process, was ground and sieved to a particle size fraction < 150 μm, and activated in a fixed bed reactor under CO2 and steam, respectively. The effect of temperature (750 to 1050 °C), reaction time (0.5 to 4 h for steam activation, 1 to 6 h for CO2 activation) and activation agent concentration (33.3 to 66.7 vol% of CO2 or steam in N2) on the carbon conversion and reaction rate was measured. The activated chars were characterised for the BET surface area, pore volume and average pore size of the activated chars using N2 adsorption and morphology using SEM. Higher temperature and activation agent concentration, and longer reaction time led to higher carbon conversion. As the carbon conversion increased, the BET surface area initially increased linearly and then decreased, reaching a maximum surface area of 666.6 m2 g−1 (0.60 conversion) for steam and 434.5 m2 g−1 (0.52 conversion) for CO2. Micropores were created in the early stage of activation, increasing first until carbon conversion reaching ca. 0.30. Steam-activated chars showed higher BET surface areas than CO2-activated chars at the same carbon conversion. Steam was found to generate both greater microporosity and mesoporosity than CO2. Graphic Abstract: [Figure not available: see fulltext.].