In this paper, the two step synthesis of tert.-butyl peroxy pivalate using two different flow regimes is introduced, in particular the use of "segmented flow" and the concept of "dispersed flow". The use of the segmented flow process, in this case, is challenging due to the very low interfacial tension of 4 mN/m at room temperature between potassium tert.-butyl peroxide and pivaloyl chloride, which causes an unstable fluid interface and results in a broad segment length distribution. This low interfacial tension is further decreased by the heat of reaction (126 kJ/mol pivaloyl chloride [1]), which is partially released at the contacting point of both fluids. It is investigated to compensate the occurring instabilities by dilution of both fluids, resulting in a more regular droplet formation. The applicability of the dispersed flow concept employing a mixer-tube set-up to create a large interfacial area for the synthesis of tert.-butyl peroxy pivalate is discussed together with the influence of increased process temperature, as a Novel Process Window, on the reaction performance. An approximate benchmark against two processes described in a patent [2] is given to point out the benefits obtained, e.g. higher space time yield, using a pre-stage micro-reactor device. (C) 2010 Elsevier B.V. All rights reserved.