Abstract: A method of starting up a Fischer-Tropsch reaction in a system comprising at least one high shear mixing zone and a reactor vessel which method comprises a) passing a suspension of a particulate Fischer-Tropsch catalyst in a liquid medium through the high shear mixing zone(s) into the reactor vessel and recycling at least a portion of the suspension to the high shear mixing zone(s) in the substantial absence of a gaseous reactant feed stream comprising synthesis gas; b) increasing the temperature and/or pressure within the reactor vessel until a threshold temperature and/or pressure is reached and subsequently introducing a gaseous reactant stream comprising synthesis gas into the high shear mixing zone(s) where the gaseous reactant stream is mixed with the suspension; c) discharging a mixture comprising synthesis gas and the suspension from the high shear mixing zone(s) into the reactor vessel; d) converting the synthesis gas to liquid hydrocarbons in the reactor vessel to form a product suspension comprisin