This paper reports an analysis of the parallelized production of bubbles in a microreactor based on the repeated break-up of bubbles at T-junctions linked in series. We address the question how to design and operate such a multi-junction device for the even distribution of bubbles over the exit channels. We study the influence of the three primary sources leading to the uneven distribution of bubbles: (1) nonuniformity in the size of bubbles fed to the distributor, (2) lack of bubble break-up, and (3) asymmetric bubble breakup caused by asymmetries in flow due to fabrication tolerances. Based on our theoretical and experimental analysis, we formulate two guidelines to operate the multi-junction bubble distributor. The device should be operated such that: (i) the capillary number exceeds a critical value at all junctions, Ca > Ca-crit, to ensure that all bubbles break, and (ii) the parameter (l(s)/w) . Ca-1/3 is sufficiently large, with l(s)/w the distance between the bubbles normalized by the channel width. More quantitatively, (l(s)/w) . Ca-1/3 > 2 for fabrication tolerances below 2%, which are typical for devices made by soft lithography. Furthermore, we address the question whether including a bypass channel around the T-junctions reduces flow asymmetries and corresponding nonuniformities in bubble size. While bubble nonuniformities in devices with and without bypass channels are comparable for fabrication tolerances of a few percent, we find that incorporating a bypass channels does have a beneficial effect for larger fabrication tolerances. The results presented in this paper facilitate the scale-out of bubble-based microreactors. (C) 2013 Elsevier B.V. All rights reserved.