The fluidized bed gasification of untreated and pre-treated olive residue and pre-treated olive residue mixed with reed, pine pellets and Douglas fir wood chips is studied. Leaching is used as a pre-treatment process targeted on the elimination of alkali metals such as K and Na as well as chlorine to reduce/eliminate the ash-related problems during gasification. The leaching pre-treatment process could affect the producer gas composition toward the lower or higher yield of CO and H-2 of the producer gas depending on the moisture content of parent fuels. The lower total tar yield of the producer gas in the case of leached olive residue was observed compared to untreated olive residue. At the same time, there are present wider varieties of different tar components in the producer gas of the leached olive residue compared to the untreated one. The distinctions in tar composition and content between the leached and untreated olive residue are attributed to the alkali and alkali earth metal and chorine chemistry affected by leaching pre-treatment. The addition of woody fuels and reed at elevated proportions resulted in the lower LHV value compared to the leached olive residue. The tar content of the producer gas is seen to increase adding reed and woody fuels to the leached olive residue, i.e. the producer gas contained additional variety of tar components whereas phenol becomes one of the key components determining the total tar content, apart from benzene, toluene and naphthalene. This is seen to be due to the higher cellulose, hemicelluloses, lignin as well as higher chlorine content of the reed and woody fuels compared to the leached olive residue. The olive residue is seen to be better fuel for gasification compared with woody fuels and reed. Even more, we believe that the leached olive residue is better compared to all other tested fuel/mixtures in this study. It is seen that the proportions of different fuels in the mixture play role in the composition of the producer gas. (c) 2012 Elsevier Ltd. All rights reserved.