The impact of ammonia in Fischer Tropsch synthesis was studied using iron- and cobalt-based catalysts. Ammonia was co-fed to the synthesis gas feed with ammonia concentrations varying between 0 and 26 vol%. During conventional Fischer Tropsch experiments the formation of paraffins, alpha-olefins, alcohols and carboxylic acids was observed. Upon ammonia addition, the selectivity to alpha-olefins could be significantly increased at all levels of ammonia addition. The conversion of CO was only affected slightly for low NH3 contents, but decreased rapidly at high NH3 concentrations in the synthesis gas feed. The formation of oxygenated products was suppressed in the presence of ammonia and the formation of nitrogen-containing compounds such as amines and amides was observed, suggesting a reaction of ammonia with oxygen containing compounds or surface intermediates. (C) 2016 Elsevier B.V. All rights reserved.