The optical fiber probe has been for the first time applied to investigate the hydrodynamics and gas-phase distribution at high gas/liquid ratios in a two-phase flow monolith bed with 0.048 m diameter and 400 cpsi. Local hydrodynamic parameters including gas holdup, bubble frequency, bubble velocity, and bubble length in single channels were measured by 16 inserted single-point optical fiber probes within the bed under a nozzle as the liquid distributor. The following findings are reported. (1) The optical fiber probe can be used as an efficient and convenient technique for measuring local hydrodynamic parameters inside the channels of a monolith bed; (2) within the range of high gas/liquid ratios under which experiments were conducted, churn flow regime occurred. In this regime, the monolith bed radial distribution of gas holdup, bubble frequency, bubble velocity, and bubble length is nonuniform in nature. (c) 2013 American Institute of Chemical Engineers AIChE J 60: 740-748, 2014