We report on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air/water interface as a function of the subphase pH and for several cations (Cd2+, Ca2+, Mg2+, and Na+) with the help of the polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) method. The infrared spectra give access to the relative concentration of acid and salt molecules and allow us to determine the influence of the subphase pH on the acid dissociation reaction for each cation. It is shown that Na+ obeys the purely electrostatic Gouy-Chapman theory quite well, whereas the behavior of Mg2+, and even more so that of Ca2+ and Cd2+, requires the introduction of some complexation constant to be understood.