Mixing can have a significant effect on the quality of the product in any fast polymerisation process, notably molecular weight distribution (MWD) in homopolymerisations. Here attention is given to the measurement of the mixing quality required to avoid reactor conditions influencing the MWD for a very fast living anionic polymerisation carried out in a flow reactor system which embodies a mixing device for the reactants as they are introduced to the reactor. The mixing measurements have been carried out using a well-established two-stage reaction, which has been shown to be suitable for the purpose of assessing mixing quality. The key variables known to influence the Reynolds number have been explored, i.e. geometry of the mixing device, reactant flow rates and reactant viscosity. Deterministic modelling methods have been used to describe the processes studied and computational fluid dynamics (CFD) methods have been used to investigate the spatial flow behaviour of the reactor system. The approach adopted is generic and might be employed for many other fast polymerisation processes.