The protonation macroconstants (log K) of 4(3H)-quinazolone (1) and two 2-methyl-4-oxo-3H-alkyl-quinazoline-3-carboxylic acid derivatives (2,3) were determined by pH-potentiometry. The acid-base chemistry of compounds 2 and 3, where proton-bindings take place in an overlapping fashion, was described in terms of protonation microconstants as well. Microspeciation was carried out by two means: UV-pH titration (selective, pH-dependent monitoring of the N1-binding site), and deductively (using a derivative compound as covalently fixed model of one of the protonation isomers). The microconstant values obtained by the two different methods are in agreement within 0.05 log K units. Microspeciation revealed remarkable differences between the two homologue compounds (2 and 3). The microconstant values show that insertion of a second methylene moiety into the aliphatic acid side-chain (1) increases the electron-density and most basicity parameters of both functional groups; (2) significantly modifies the extent of site-site interactions in the molecule; (3) opens new conformational preferences by N1 ring nitrogen-carboxylic group intramolecular hydrogen bond formation and (4) reverses the predominantly zwitterion-involved protonation pathway into a neutral form-involved pathway. These molecules exemplify that microconstant values allow the comparative prediction and quantitative evaluation of pharmacokinetic behaviour, and signify the fact that microspeciation is a powerful tool in the process of drug development.