Bubble column reactors are used in industrial practices due to their intrinsic advantages of good mixing ability, high heat transfer and operational versatility. Generation of small bubbles in bubble column is a crucial step to improve their performance. The present investigation introduces a new approach for bubble breakup in an upward co-flowing liquid using a honeycomb monolith breaker with square cell structure. The experimental measurements were conducted using high speed imaging at different superficial liquid velocities and gas flow rates ranging between 8 to 50 cm/s and 165 to 1000 ml/min, respectively. A comparison between the bubbles generated from the monolith breaker and those generated from the nozzle shows that the monolith breaker reduces the bubble size by approximately 60% over the given range of liquid superficial velocities and gas Row rates. It is observed that at low superficial liquid velocities and low gas Row rates, the bubble size at the breaker exit follows log-normal distribution, which becomes more symmetric at higher superficial liquid velocities and gas Row rates. The main contributor of large bubbles formation at the monolith breaker exit is the bubbles' coalescence. Different mechanisms of bubbles coalescence at the breaker outlet are observed and classified into three types; multi- and successive ''accumulative" coalescence, multi- and non-successive coalescence, and bubbles coalescence in the vicinity of the breaker outlet. The efficiency of the breaker is quantified in terms of the fractional conversion of bubbles' kinetic energy into the surface energy. A strong dependency of the breaker efficiency on the superficial liquid velocity is observed. The results indicate that an optimal liquid velocity exists that corresponds to the minimal bubble coalescence at which the breaker efficiency is maximum. (C) 2015 Elsevier Ltd. All rights reserved.