<p>This work is a continuation of NMR studies of the factors that affect the solubility of hypromellose phthalate, which is used as the main component of enteric shells and provides controlled release in the small intestine of the active substance in solid dosage forms. The purpose of the research was to use NMR methods to study the dissolution profiles of hypromellose phthalate in aqueous buffer solutions with pH 4.0, 4.5, and 6.8 considering differences in the polymer structure. Hypromellose phthalate samples from different manufacturers were investigated. Dissolution profiles were studied using a PMR method; nonstoichiometric polymer compositions, a ¹³C NMR method; and molecular masses, diffusion-ordered NMR spectroscopy. Hypromellose phthalate samples with molecular masses in the range 19 – 29 kDa and phthalate contents 24 – 35% were stable in solutions at pH 4.0 and 4.5 over a controlled time range (10 – 90 min). The minimum solubility was observed for the sample with the highest molecular mass in solution at pH 6.8 with similar phthalyl contents in the polymer molecules. A hypromellose phthalate sample with molecular mass 16.07 kDa was stable in solution at pH 4.0 but dissolved at pH 4.5. A hypromellose phthalate sample with molecular mass 10.79 kDa dissolved at pH 4.0. Thus, average molecular mass of hypromellose phthalate was an important parameter affecting its solubility. Polymers with an average molecular mass of at least 19 kDa were stable in solutions at pH ≤4.5. Hypromellose phthalate with molecular masses below 17 kDa dissolved in these media. It seemed reasonable to evaluate the average molecular mass of hypromellose phthalate in quality control. Dissolution medium with pH 4.5 must be included in pharmacopoeial dissolution tests of gastro-resistant drugs for the treatment of acid-dependent gastric diseases.</p>