This article describes the proper selection of extraction solvents to eliminate interference from a polymer matrix to the quantitation of estradiol degradation products in a transdermal formulation by reversed-phase liquid chromatography. The separation is performed by gradient elution with acetonitrile and water as the mobile phase on Inertsil ODS columns. Severe band distortion and insufficient recovery are observed for two geometric degradation products (or impurities) when the sample is prepared by acetonitrile. It is anticipated that the poor resolution and recovery are caused by multiple retention processes due to the reversible binding of degradation products to the polymer matrix (or the impurity-polymer interaction). This interaction is eliminated by adding formamide, a solvent that possesses similar properties to the matrix, in the extraction solvent. It is believed that the favorable interaction between formamide and the polymer matrix releases the impurity molecules, and they can then be separated by a single retention mode. It has been confirmed experimentally that the use of formamide in the extraction solvent not only sharpens the peaks tremendously, but also recovers the degradation products completely.