In this work an ultrafast electron beam X-ray modality was applied for the first time to characterize the gas-liquid Taylor flow inside each channel of an opaque honeycomb monolith structure ( 65cpsi) for uG,S=0.1...0.5m/sanduL,S=0.2m/s. Significant spatial and temporal deviations in the phase holdup as well as in the gas bubble and liquid slug lengths were found. To evaluate the impact of Taylor flow maldistribution on the reactor performance, the data of more than 125,000 unit cells were used to simulate the reactor productivity in the hydrogenation of glucose. The results verify that a monolith reactor solely designed by using superficial velocities and empirical correlations for gas bubble and liquid slug lengths fails significantly in achieving high product selectivity and the desired conversion. The developed methods are a solid base to design and select proper distributors ensuring the favorable flow configurations for specific chemical processes. (c) 2016 American Institute of Chemical Engineers AIChE J, 62: 4346-4364, 2016