Microemulsion systems consisting of water, oil and non-ionic surfactants can develop up to three liquid phases as a function of composition and temperature. Under three phase conditions, Population Balance Equations (PBEs) derived from two phase systems need to be extended to account for the influence of the third phase on the droplet size distributions. PBE fitting parameters can not be simply transferred from two phase to three phase conditions, which is attributed to the changing composition of the liquid phases induced by solubility variation. A method to derive PBE fitting parameters for the three phase conditions and describe the droplet sizes in these systems is presented herein.