Current knowledge on the design and performance of three-phase monolithic reactors and mini-structured reactors is limited by a lack of knowledge on hydrodynamics and mass transfer while a chemical reaction takes place. Therefore, a new method for the simultaneous measurement of hydrodynamics and reaction rates in a mini-channel with catalytically active walls at pressures up to 5 MPa is presented. A newly developed setup has led to an investigation of the influence of the operating conditions and of the gas-liquid feeding on the characteristics of Taylor flow and on chemical conversion. The detected parameters include gas bubble length, liquid slug length, bubble flattening in the cross-sectional area, change in the gas bubble velocity due to conversion and overall macrokinetics. The hydrogenation of alpha-methylstyrene was used as a test reaction system at a pressure of 1 MPa and a temperature of 343 K. (C) 2011 Elsevier B.V. All rights reserved.