This invention relates to a high temperature Fischer-Tropsch (HTFT) hydrocarbon synthesis process comprising the conversion of a feed of H2 and at least one carbon oxide to hydrocarbons containing at least 30% on a mass basis hydrocarbons with five or more carbon atoms (hereinafter referred to as C5+ compounds). The conversion is carried out in the presence of an alkali-promoted iron hydrocarbon synthesis catalyst, and the process is characterised therein that the reaction mixture formed during the conversion contains less than 0.02 mol alkali per 100 g iron, and that the H2:carbon oxide molar ratio in the feed of H2 and carbon oxide is at least 2.