Paper presents the experimental investigation of a sewage sludge gasification process. Installation with an updraft gasifier was used. An analysis of how sewage sludge composition, volatile matter content, and water content influence the composition of the gas obtained in the autothermal gasification process was conducted. The results were presented as a function of the amount of gasification agent and show that higher oxygen content in sewage sludge is detrimental to the temperature of the reaction. Paradoxically, this results in an increase in the quantity of combustible components in the gas. As expected, an increase in the air flow rate causes a decrease in the heating value. Greater amounts of oxidizing agents tend to increase the amounts of noncombustible species and the volumetric fraction of nitrogen, thus decreasing the heating value of the gas obtained.