The pK_a values of propanolamine hydrochloride (PAMH) and poly(allylamine hydrochloride) (PAAMH) in concentrated solutions were determined by both Fourier transform infrared spectroscopy (FT-IR) titration and classical potentiometric (POT) titration and compared. Starting with the respective fully protonated forms PAMH and PAAMH and increasing the pH value by sodium hydroxide addition in situ attenuated total reflection FT-IR (ATR FT-IR) spectra on PAMH and PAAMH solutions show the variation of diagnostic infrared (IR) bands. From the decrease of the most intense δ(NH₃⁺) band the dissociation process of the NH₃⁺ groups could be followed. Thereby, from the respective normalized band area A the dissociation degree α_IR of the ammonium groups could be determined. Plotting pH versus α_IR and fitting this curve by a modified Henderson–Hasselbalch function pH = pK_a + B log (α_IR/1 – α_IR) the parameters pK_a and cooperativity factor B were obtained. pK_a values from FT-IR titration were qualitatively in line with respective pK_a values from POT titration. Quantitative systematic pK_a deviations between polyelectrolyte (PEL) and respective monoelectrolyte and the tentative effects of PEL molecular weight, ambient ionic strength, and titration concept (FT-IR and POT) are discussed based on classical models of weak PEL.