In internal-mixing twin-fluid atomizers the liquid is entrained by an airflow of comparatively low velocity. When passing the orifice of the atomizer the gas and the liquid are accelerated simultaneously and the liquid is atomized when the Weber number We increases up to a certain limit. Experiments with single drops show that both the geometry of the nozzle and the viscosity of the liquid influence the onset of the liquid breakup and the Sauter mean diameter D-32 of the secondary spray. This influence on both the onset of liquid breakup and the characteristic D-32 of the spray has recognized even for low Ohnesorge numbers On < 0.01. An optimum Ohnesorge number has been found for which the bag breakup occurs for lowest Weber numbers and the Sauter mean diameter of the secondary droplets becomes minimum. Within the range examined, the Sauter mean diameter reaches a minimum for We < 40 and only insignificantly increases at Weber numbers up to We less than or equal to 250.