The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pK(a) ) of the protic functional group(s) present. Here, we use three independent methods-compression isotherms, surface tension pH titration, and infrared reflection-absorption spectroscopy (IRRAS)-to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pK(a) to remove the second proton of DPPA (surface pK(a2)) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pK(a2) values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pK(a2) of 11.5, a value higher than previously reported (similar to 7.9-8.5). This surface pK(a2) was further altered by the presence of Na+ cations in the aqueous subphase, which reduced the surface pK(a2) from 11.5 to 10.5. It was also found that the surface pK(a2) value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pK(a2) as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pK(a2) values are also found to be in agreement with those predicted by Gouy-Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pK(a2).