In this paper, we present detailed experimental and modeling studies of a recently developed triphasic segmented flow millireactors for rapid nanoparticle-catalyzed gas–liquid reactions. We first present detailed observations of the hydrodynamics and flow regimes in a pseudo-biphasic mode of operation, which enable the design and selection of optimal operating conditions for the triphasic millireactor. We particularly focus on and analyze the presence of wetting films of the organic phase on the reactor walls at high flow speeds, a consequence of the phenomenon of forced wetting, which is a key ingredient for optimal reactor performance. Next, we describe the development of a simple phenomenological model, incorporating the key mass transport steps that accurately captures the observed experimental trends for the rhodium nanoparticle (RhNP) catalyzed hydrogenation of a model substrate (1-hexene). We further discuss and analyze the consequences of this model.