The flow characteristics of a miniemulsion in a tubular reactor were assessed using a modified dye tracer approach. Metered nitrogen was introduced producing isolated plug flow. The effects of the flow regime were then related to the polydispersity of polystyrene formed using reversible addition fragmentation chain transfer (RAFT). While nonideal flow was exhibited at very low Reynolds numbers (<= 20), laminar flow was not. This was attributed to fluid slippage at the wall of the reactor. It was demonstrated that the flow regime and residence time distribution have a direct effect on the molecular weight polydispersity of the polymer. Transient behavior during startup of isolated plug flow was attributed via experimental data and modeling to loss of initiator by way of water droplets left behind by the small cylinders of miniemulsion. (c) 2006 American Institute of Chemical Engineers.