Pyrolysis of sewage sludge in the presence of humic acid (HA), CaO and kaolin was investigated in order to examine the effects of organic and inorganic constituents on the product properties. The presence of HA in the sludge increased the yields of H₂ and CO compared to that from the raw sludge pyrolysis. Water evolved during decomposition of kaolin promoted the water-gas shift reaction resulting in less CO in the gas phase. The presence of CaO in the sludge had a major influence on CO₂ sequestration reducing the volume of non-condensable gas by 18% compared to that from the raw sludge pyrolysis. In contrast, CO volume increased due to the induced Boudouard reaction. No significant impact on the distribution of the chemical compounds in the organic or aqueous phase was observed with the addition of HA to the sludge. Among the inorganic additives, CaO catalysed deoxygenation and ring opening reactions leading to an increment of aliphatic compounds in the organic phase of tar while kaolin did not show any significant impact. HA in sludge increased the carbon content in the derived char to 35.2% compared to that in the char derived from the raw sludge (31.0%). In contrast, kaolin and CaO resulted in higher char yields with lower carbon contents due to high ash content in the raw samples. Regarding heavy metals fate, the addition of HA and CaO to the sludge effectively immobilised heavy metals in the derived char while kaolin appeared to be less effective.