The influence of an endogenous surfactant present in Eudragit NE30D on the structure and drug release (clenbuterol) properties of thin matrices has been examined. Both drug-free and drug-loaded matrices were found to be non-isotropic in structure, the former having a marbled appearance under the polarising light microscope, and the latter showing numerous needle-shaped crystals. At loading above approx. 10% w/w clenbuterol it was also possible to observe aggregates of the drug. Differential scanning calorimetry enabled the identification of melting peaks at approx. 50°C for the needle-shaped crystals and approx. 80°C for the larger drug aggregates. The former are composed of a surfactant used by the manufacturer for the synthesis of Eudragit NE30D by emulsion polymerization. This surfactant undergoes a phase separation from the polymer on storage at room temperature. It could, however, be extracted from the polymer by refluxing in water to yield an isotropic system. The extract showed a melting peak at 50°C and also UV, IR, NMR, and mass spectra in accordance with an o-substituted nonyl phenol surfactant. Matrices prepared from the purified Eudragit NE30D showed drug release rates of only one third the magnitude of those found with matrices prepared from the raw polymer. Substantially reduced scatter in the release data was also found with the purified polymer.