Biomass gasification to generate raw syngas used in anaerobic fermentation processes is one of several emerging technologies for the production of biofuels from biomass. The gasification-fermentation process can utilize a wide variety of lignocellulosic biomass such as prairie grasses, wood chips, and paper wastes, in addition to non-lignocellulosic biomass such as solid municipal wastes. Although the primary components of raw syngas used in the fermentation process are CO, H-2, and CO2, several impurities can also be present. Some of these impurities may interfere with the fermentation process. Since the impurities will depend upon the feedstock, the gasifier type, and cleanup conditions, an understanding of the positive or adverse effects of the impurities on raw syngas fermentation is critical to understand the need for efficient gas cleaning processes required for commercialization. This work describes the impurities generated during gasification and discusses the potential accumulation of impurities in the fermentation media and the associated potential effects on the microbial fermentation process (e.g. cell toxicity, enzymatic inhibition and end product distribution). A wide distribution of impurity solubilities in the media shows that certain impurities, such as ammonia, are more likely to accumulate in the media. Additionally, entrained tar particulates greater than 0.025 mu m, nitric oxide greater than 0.004 mol%, and ammonia in general have an adverse effect on the fermentation process. Therefore, a cleanup system suitable for raw syngas fermentation processes is evident although the degree of cleanup would likely depend upon the feedstock and the associated gasification process. (C) 2011 Elsevier Ltd. All rights reserved.