Abstract Hollow-fiber emulsification is a recent development that allows a continuous production of narrow-dispersed droplets with tunable diameter using hollow-fiber membranes. The to-be-dispersed phase is fed on one end through the lumen side while the continuous phase permeates through the membrane wall. After droplet formation, a droplet train leaves the hollow fiber on the other end. Droplet break-up has been assumed to occur inside a hollow-fiber membrane originating from Rayleigh-like instabilities, however proof has been missing yet due to missing visualization techniques inside the hollow fiber. Here we proof for the first time experimentally the droplet break-up mechanism and support these findings using computational fluid dynamics (CFD) simulations. Results of the CFD simulations are compared with experiments carried out using a glycerol-water solution as droplet phase and paraffin oil as continuous phase. The oil turns the porous hollow-fiber membrane transparent thus allowing direct observation of the droplet formation process inside the membrane. Comparisons were carried out by variation of the volume flow rate of the disperse phase and its viscosity as relevant influencing parameters and for the target parameters droplet size and droplet break-up length. Simulations capture the governing trends of the process such as the presence of a size maximum and allow the prediction of the required membrane lengths for droplet break-up. The droplet size maximum is explained using the understanding of droplet detachment in co-flowing streams.