Residence time distributions (RTDs) were investigated in a retention reactor (RR) and a coaxial heat exchanger (CHE) of a commercial modular micro reaction system, to improve processing time control and implement this system as a lab scale tool for thermal process development. RTD were investigated by step experiments (30 g/L NaCl solution against distilled water at 20 degrees C). Given a measured in- and outlet conductivity distribution where the outlet was produced by a convolution with a transfer function E(t), E(t) parameters (mean residence time theta and variance alpha(2)) were recovered using a computational routine. The flow in the RR remained close to a plug flow despite laminar conditions. Molecular diffusion was not sufficient to explain this improvement and further modeling would be required to clarify this behavior. The CHE showed a significant dispersion and 0 two to five times longer than estimated, possibly due to flow defects associated to its complex geometry. (C) 2013 Elsevier Ltd. All rights reserved.