This is the study of environmental implications of sewage sludge cogasification mixed with biomass wastes (straw pellets). NH3, H2S, and HCl contents of syngas were compared to those produced during cogasification of sewage sludge with coal. In the presence of straw pellets, lower concentrations of NH3 and higher contents of HCl were obtained, probably due to its low and high contents of N and Cl, respectively. However, NH3 and HCl contents in the syngas were very low and similar for both sewage sludge blends with straw pellets or with coal, because most of these compounds were retained in the condensation system of the gasification installation. Lower H2S levels were also measured when blends of sewage sludge with straw pellets were used, probably due to the lower sulfur contents of straw pellets. The results obtained show that it is possible to cogasify sewage sludge with either biomass, such as straw pellets, or coal, without major modifications in previously existing gasification installations. However, because the release of undesirable S, N, and Cl compounds is most affected by the contents of these elements in the fuels, special care should be taken when substituting one fuel for another and an adjustment of experimental conditions is most advisable. The results obtained showed that sewage sludge should be cogasified with straw pellets to take profit of its diluting effect, although effective sorbents could be added to the gasification medium to achieve further reductions of sulfur compound release. Gasification solid residues were analyzed to evaluate possible reuses and leachability behavior. The leachability tests performed according to European regulations showed that metals leachability was within the limits for landfilling inert residues, but chlorine release could be very high, especially in the presence of straw pellets, due to its high chlorine content.