This work analyzes flow regimes and mass transfer in a miniaturized packed bed reactor under reacting conditions. The reactor consisted of a cylindrical channel with an inner diameter of 2 mm which was filled with spherical catalyst particles having an outer diameter of 0.8 mm. The liquid phase hydrogenation of alpha-methylstyrene over a Pd/gamma-Al2O3 catalyst was employed as reaction system. Based on experimental reaction rates, overall (gas-liquid-solid) volumetric mass transfer coefficients of hydrogen, which were between 0.5 s(-1) and 6.0 s(-1), were computed. Detailed discussions focus on the impact of the flow regime, the gas and liquid superficial velocities and, consequently, their ratio, as well as system pressure on overall mass transfer rates. (C) 2013 Published by Elsevier B.V.