The effect of co-fed ammonia in syngas on the performance of a traditional cobalt catalyst (platinum promoted cobalt/alumina) was investigated during Fischer-Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR). The addition of 1, 10,100, and 1200 ppmw (concentration of ammonia with respect to the syngas feed) of ammonia resulted in significant irreversible deactivation of the catalyst under identical reaction conditions. The percentage of deactivation was almost constant (+/- 5%) at all the concentrations (1-1200 ppmw) of ammonia exposure. The addition of ammonia (either NH3 gas or aqueous NH4OH) particularly at higher concentrations exhibits lower methane selectivity compared to ammonia-free synthesis conditions. This enhancement in C-5+ hydrocarbon formation may be attributed to the selective poisoning of metallic cobalt sites by addition of ammonia. (C) 2013 Elsevier B.V. All rights reserved.