<p>Several drugs with coatings based on methacrylic acid-ethyl acrylate copolymers, which should be stable in acidic medium, were experimentally found to dissolve at pH 4.5. The non-stoichiometric composition and microstructure of these methacrylic acid-ethyl acrylate copolymers with different solubilities were comparatively analyzed by NMR to understand the reason for this phenomenon. Astructural interpretation of the PMR and ¹³C NMR spectra of methacrylic acid-ethyl acrylate copolymer was presented. The mass fractions of methacrylic acid were estimated by PMR for 10 samples; the microstructure of the copolymers, by ¹³C NMR. The different dissolution profiles of these samples were found not to be related to their structural specifics. It was suggested that the instability of the drug coating at pH 4.5 was due to a noncompliance of the technological process for its application or a deviation of the average copolymer molecular mass from the nominal value.</p>