Recently, we proposed microchannel (MC) emulsification, a novel method for making monodispersed emulsion with a coefficient of variation (CV) less than 5%. In this study, we attempted monodispersed microbubble formation using the MC technique. Monodispersed microbubbles were formed using surfactants and proteins as dispersing agent. The average bubble size ranged from 33.6 to 51.1 mum, and CV values were below 10%. The bubble formation behavior was compared with the droplet formation behavior. The microbubble formation appears to be based on the spontaneous transformation caused by surface tension, as previously shown for liquid-liquid systems. The average bubble size was affected by viscosity, which is consistent with the results for the liquid-liquid system in previous studies. In the case of protein, the effect of surface viscoelasticity was indicated, which was not reported for the liquid-liquid system. (C) 2004 American Institute of Chemical Engineers.