A reactor module for Fischer-Tropsch synthesis having a generally rectangular reactor block with a stack of plates defining flow channels for coolant and flow channels for the synthesis reaction arranged alternately in the block. The synthesis flow channels extend in a generally vertical direction between upper and lower faces of the reactor block and are defined by plates in combination with either bars or sheets such that each channel is of width no more than 200 mm. The coolant flow channels are oriented in the same direction, and communicate through distributor chambers with inlet and outlet ports at side faces of the reactor block. A plant may contain a multiplicity of such reactor modules operating in parallel, the modules being interchangeable and replaceable. The temperature control is enhanced by allowing the coolant flow to be parallel to the synthesis gas flow.