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In this study, we demonstrate a mesoscale triphasic (gas–liquid–liquid) reactor for fast, transition metal catalyzed gas–liquid reactions, which is capable of delivering kg per day productivity at the single channel level. More generally, our study addresses the limits of scale up of multiphase flow reactors beyond the micro- and milli-scale. We first conduct a rigorous hydrodynamic study that allows us to explore the channel dimension and reactor operating conditions within which a stable and regular flow regime can be maintained. We particularly focus on the presence of the organic phase as a thin film around the train of dispersed phase segments, since this plays a key role in process intensification and flow stability. A tube diameter of 3.2 mm is found to be the upper limit for the mesoscale channel, beyond which thin films cease to exist due to combination of gravitational drainage and dewetting. Next, we present experimental observations of a model reaction – the hydrogenation of 1-hexene in the presence of a rhodium nanoparticle catalyst (RhNP) to evaluate the reactor performance and highlight the key differences between micro/milli-scale and mesoscale operation. Finally, we develop and discuss a mathematical model that accurately captures the key experimental observations. Based on the insight we gain from our model, we demonstrate further scale up of the reactor to achieve the performance of >100× equivalent milliscale flow reactors with a single mesoscale channel under ambient conditions.