Gas jet penetration is an important factor in gas distributor design for two- and three-phase reactors. The penetration length of downward nonreacting gas jets was measured it? liquid by optical method and for gas jet velocities at subsonic conditions. The effects on jet penetration of nozzle diameter, gas density, gas jet velocity, liquid viscosity, and liquid surface tension were examined. A generalized correlation to predict the penetration depth of downward gas jets was obtained. Also a simple model, using a shell momentum balance and assuming downward gas jet as a downward cone with constant angle, was introduced and validated with experimental data. Bubble size calculated based on gas jet fluctuation frequency and validated by high-speed videos. (C) 2008 American Institute of Chemical Engineers.