In this work, we used polyoxyalkylenated diethylenetriamine (DETA) demulsifier with various polyethylene oxide (PEO) and polypropylene oxide (PPO) contents to destabilize a stable water-in-oil emulsion. The demulsifiers were characterized by the relative solubility number (RSN). The efficiency of emulsion destabilization was measured by the degree of separation of the oil and water phases. It was found that the destabilization of an emulsion is closely correlated with the PO and EO numbers. When the PO number in a molecule is much greater than the EO number, the surfactant gives a very low oil resolution rate, requires high dosages, and produces a stable middle phase. When a surfactant contains more EO than PO, it gives a high oil resolution rate at a low dosage, but it is easily overdosed, and some surfactants of this type (high molecular weight) also produce a stable middle emulsion phase. When the PO and EO numbers in the surfactant are close to equal, the surfactant breaks the emulsion rapidly at a very low dosage, does not show overdosing at very high dosage, and does not produce a stable middle phase. Therefore, surfactants with balanced PO and EO numbers give optimal performance.