The invention relates to reactor apparatus (1) comprising an assembly of a plurality of separate conduits (2) disposed within a vessel (3) for heat exchange between the conduits (2) and a medium (not shown) in the vessel (3), the separate conduits (2) being connectible to define one or more flow paths through the reactor (1), the length of the or each flow path being variable by adjusting the number of conduits connected such that the residence tine of reactants flowing in the or each flow path can be varied, and a mixing inlet (100) for mixing fluids comprising a conduit (200) adapted to be inserted into a fluid flow device (300) and means (400) disposed about the outer surface (700) of the conduit (200) to create turbulence in fluid in the device (300), there being at least one aperture (600) in the conduit (200) for addition and an additive, the turbulence causing mixing of the additive into the fluid show. Such a mixing inlet (100) can be used with reactor apparatus as described above.