Straight-through microchannel (MC) emulsification is a novel technique for producing monodisperse emulsions using a silicon array of micromachined through-holes, named a straight-through MC. This paper investigates the effect of surfactants on the straight-through MC emulsification behavior. The oblong straight-through MC used in this study had channel lengths of 48.7 and 9.6 mum. Using anionic and nonionic surfactants, monodisperse soybean oil-in-water (O/W) emulsions with average droplet diameters of 39 mum and coefficients of variation below 3% were successfully prepared. In contrast, the systems containing cationic surfactants resulted in the preparation of polydisperse O/W emulsions and complete wetting of the dispersed phase on the plate surface. An analysis of the above straight-through MC emulsification behavior revealed that a repulsive surfactant-plate surface interaction and a high contact angle value are required to achieve successful straight-through MC emulsification. Analysis also revealed that it is important to keep the negatively charged plate surface hydrophilic during the emulsification process. The dispersed-phase flux affected the straight-through MC emulsification behavior. A size-stable zone, a size-expanding zone, and a continuous outflow zone were observed with an increase in the dispersed-phase flux when using anionic and nonionic surfactants. (C) 2003 Elsevier B.V. All rights reserved.