pH modulation of aqueous mixture samples combined with FT-IR detection and a powerful second-order resolution method is proposed for both resolution and quantitation of acid analytes in the presence of similarly behaving interferences. The proposed method allows for the analyte determination in mixtures using a single standard sample per analyte. Due to the very similar pKa values of the investigated analytes and interferences, the highly correlated concentration profiles of these compounds cannot be successfully resolved with pure soft-modeling second-order approaches. The inclusion of a hard-modeling constraint based on the acid−base equilibrium model in the soft-modeling curve resolution method has allowed for the unambiguous resolution of the analyte profiles and, as a consequence, for the correct quantitation of this compound in the mixture sample. A detailed discussion of the combined hard−soft-modeling approach as well as the analytical problem and the quantitation results is given. Also, strategies to overcome problems associated with variation in pKa values between different samples are addressed. Due to the flexible implementation of the hard-model equilibrium constraint in the multivariate curve resolution-alternating least squares method, this approach is expected to be useful also for analysis of other complex mixed equilibrium-based chemical systems.