The mechanism of emulsifier blending of different hydrophile-lipophile balance has been studied by the measurements of 1) the phase inversion temperature ( PIT) in emulsions, 2) the size distribution of droplets and the stability of emulsions, and 3) the cloud points of aqueous solutions. The PIT was slowly depressed with the replacement of lipophilic emulsifier at first and then suddenly and discontinuously dropped when the difference in HLB of two emulsifiers is too large. These results imply 1) the HLB number of emulsifier blend markedly deviates from weight average of two emulsifiers and 2) the PIT of certain temperature range, i. e., HLB of certain range cannot be prepared by blending, provided the difference in HLB (or PIT) of two emulsifiers is too large. The emulsion droplets were smaller and the emulsion was more stable when the difference of HLB of emulsifiers to be blended is smaller in the mixture of Tween and Span type emulsifiers. There was, however, an optimum difference in HLB in the blends of polyoxyethylene nonylphenylethers. It is clear that the blending of extreme two emulsifiers yields less stable emulsions and additional emulsifier of medium HLB is necessary. In other words, an emulsifier whose solubility behavior towards water and oil is medium is necessary as well as highly water and oil soluble emulsifiers in order to prepare stable emulsions over the wide HLB range required