The Fischer-Tropsch synthesis (FTS) mechanism is still not fully understood and the product distribution does not generally follow a typical Anderson-Schulz-Flory (ASF) distribution. In order to improve the understanding of the reaction mechanism and in an attempt to explain the deviations observed from the ASF distribution, additives such as olefins, alcohols, carbon dioxide, water, and isotope markers, have been used in the FTS. This paper briefly reviews the effect of additives and their impact on the FTS mechanism. Crown Copyright (C) 2011 Published by Elsevier B. V. All rights reserved.