Droplet formation of various liquid/liquid systems is investigated in an interdigital micromixer made of glass. By means of the interdigital channel configuration, cylindrically shaped or lamellae-like substreams of the dispersed phase are generated inside a continuous phase. Using the model emulsion silicon oil/water, the Rayleigh plateau instability is identified as the decay mechanism for cylindrically shaped substreams of the dispersed phase. In comparison, it is suggested by the droplet size distribution of various mixers as a function of the total flow rate that in mixers with more complex fluid-guiding geometries droplet formation is mainly affected by shear and vortex flows.