High pressure homogenization is widely used to produce small droplets in liquid products. The use of two homogenization units (e.g. orifices) in series thereby is generally accepted as increasing homogenization efficiency, even when the mechanism is still not fully understood yet It is shown that for a single stage orifice operated at a given overhead pressure, the drop size decreases with increasing backpressure first until a minimum is reached and then increases clue to the lower total pressure drop due to the lower flow velocity and driving force. Simultaneously observation showed that backpressure suppresses cavitation. This paper focuses on the question whether the droplets break up at the first or second orifice. A process plant is designed enabling us to investigate droplet breakup in o/w-emulsions at each orifice separately. In subsequent experiments the second orifice was substituted by a pressure vessel to eliminate the influence of the second orifice entirely. Our research reveals that droplets are broken up only in the first orifice. The second orifice delivers back pressure being responsible for a change in the cavitation pattern found after the first orifice. The smallest oil droplet sizes are reached at a back-pressure of similar to 30% of the inlet pressure, a value being in consensus to former experimental studies. A discussion of the droplet breakup with regard to the mean specific energy input E-v at the first orifice is given. It depicts that a change in flow pattern, especially the appearance of vapor after the first orifice has a huge impact on energy transfer efficiency. A prediction of resulting droplet sizes as result of pressure drop in the first stage is only possible for constant Th numbers. (C) 2015 Elsevier Ltd. All rights reserved.