The effect of surfactant hydrophilic-lipophilic balance (HLB) on the catastrophic phase inversion of an emulsion, and the internal structure of the drops, was studied by using a model abnormal O/W/O emulsion. The system was composed of polyisobutylene, water, and a mixture of a water-soluble and an oil-soluble surfactant. The abnormal O/W/O emulsions were made by gradual addition of water to the oil in the presence of surfactant until the catastrophic phase inversion to the corresponding normal O/W emulsions occurred. The presence of an oil-soluble component in the surfactant, even in a low quantity, enhanced the stability of the abnormal W/O emulsion. The phase inversion was slightly advanced, in terms of water volume fraction, with decreasing HLB. Some indication was found that as the optimum HLB was approached, the inclusion of continuous phase into the dispersed phase increased. The size of internal oil droplets, which were entrained in the multiple water drops, was largely reduced with decreasing HLB as the optimum formulation was approached. The size of multiple water drops, which are dispersed in the continuous oil phase, however, showed a considerably smaller decrease with decreasing HLB. When, at a fixed oil/water ratio, the HLB of the abnormal emulsion approached that of the optimum, the catastrophic phase inversion to a corresponding normal emulsion occurred, indicating that abnormal emulsions cannot exist in the vicinity of the locus of transitional inversion.