Studies on the effect of a back-pressure inducing second orifice on emulsification efficiency concentrate on its effect on the droplet sizes obtained after the second stage. At constant inlet pressure pinlet an increase in emulsification efficiency is obtained with increasing pbp. This is represented by a reduction of droplet sizes compared to a single stage process at the same pinlet. A minimum droplet size is obtained at back-pressure to inlet pressure ratios of 20–30%