Incorporation of pH modifiers is a commonly used strategy to enhance the dissolution rate weakly basic drugs from sustained release solid dosage forms. Electron paramagnetic resonance imaging (EPRI) was applied to spatially monitor pH_M and the rotational correlation time (τ_R), a parameter which is closely related to the surrounding microviscosity inside HPMC (hydroxypropylmethylcellulose) matrix tablets. Fumaric, citric, and succinic acid were employed as pH modifiers. 4-(methylamino)-2-ethyl-5,5-dimethyl-4-pyridine-2-yl-2,5-dihydro-1H-imidazole-1-oxyl (MEP) was used as spin label. Fumaric and citric acid reduced the pH_M to equal extents in the initial phase. With the progress of hydration, the more soluble citric acid diffused out from the tablet resulting in an increase in pH_M, originating at the outer layers. In contrast, fumaric acid maintained a constantly reduced pH_M inside the entire tablet. Due to its lower acidic strength, succinic acid did not reduce the pH_M as effectively as the other pH modifiers used. The more water-soluble acids stimulated the water penetration into the matrix system, thereby rapidly decreasing τ_R. Once the matrix tablets were hydrated, the included pH modifiers influenced τ_R insignificantly. EPRI, a novel approach for monitoring pH_M and τ_R non-invasively and spatially resolved, was used successfully for the optimization of an pH-controlled formulation.