The release of H2S and NH3 into syngas during co-gasification of two coals (German and Polish) mixed with wastes (pine, olive bagasse and polyethylene) was studied. Sulphur and nitrogen contents in feedstock were found to have a great influence on H2S and NH3 concentrations in syngas, as the highest contents of these elements led to the highest releases. Air/steam or oxygen/steam mixtures were used in the gasification medium, keeping constant experimental conditions, including equivalent ratio. However, when air was added instead of oxygen, higher flow rates were used, due to the introduction of nitrogen and thus lower residence times were used. Different H2S and NH3 contents were obtained by changing the gasification medium. In presence of oxygen and steam higher H2S contents and lower NH3 concentrations were obtained than those produced in presence of air and steam. However, after syngas hot treatment in two fixed bed reactors, the first one with dolomite and the second one with a Ni-based catalyst (G-90 B 5) these differences lost significance. On the other hand, different final compositions of H2S and NH3 were obtained for different feedstocks. Those with highest sulphur and nitrogen contents led to the highest final H2S and NH3 contents in syngas. (C) 2012 Elsevier Ltd. All rights reserved.