The effect of mixing chamber geometry and feeding system geometry on g/l-dispersion in interdigital micromixers was basically investigated with regard to bubble size and bubble size distribution (BSD) using the systems nitrogen/aqueous glycerol solution with sodium dodecyl sulphate (SDS) as surfactant and nitrogen/water with SDS. The process of bubble formation was visually observed in glass micromixers with different mixing chamber geometries. This was set in correspondence to bubble size and BSD of the foams finally formed. Dependencies were deduced from comparing total flow rates, flow rate ratios and mixing chamber geometries. These investigations were supplemented by similar experiments with micromixers fabricated out of metal stainless steel. Based on the insight that a multi-channel gas feeding system without special measures for equipartition of the gas is counterproductive for small bubbles and narrow BSD, a new feeding system with generic flow configuration was developed and tested for the stainless steel micromixer. The feeding system was therefore simplified from a multi-channel gas feed system to a single-channel one. The single gas feed channel was encompassed from both sides by two liquid feed channels. It was shown that with this new feeding system smaller bubbles with a narrow size distribution are formed. (C) 2004 Elsevier B.V. All rights reserved.