Slurry phase Fischer-Tropsch synthesis has been conducted starting with an ultrafine iron oxide catalyst with an average particle size of 30 angstrom. Catalyst activity gradually increased during the first 400 hour of synthesis and then remained stable for 250 days. Potassium promotion greatly increased the initial activity of the catalyst; however, the stability decreased. The CO conversion of the unpromoted catalyst was found to vary linearly with the reciprocal of the space velocity up to 60% conversion. Alkene selectivity increased with increasing space velocity but remained constant above GHSV = 2.0. Secondary reactions are believed to account for the higher alkane content at low space velocity. Methane, ethane and alkane formation increased with increasing hydrogen partial pressure up to a H-2/CO ratio of 1. CO conversion is highest at lowest CO partial pressure and likewise for hydrogen. In general it is observed that this ultrafine catalyst is not as active as precipitated catalysts and has poor selectivity for alkene synthesis.