Fourier transform infrared spectroscopy in the internal reflectance (attenuated total reflectance) mode (ATR-FTIR) has been applied to the study of carboxylate functionalized thin films. Both the monitoring of the self-assembly of an alkyl carboxylate-bearing siloxane-anchored monolayer and the use of pH difference spectra to directly assess (in situ) the acid-base properties of such a functionalized film have been achieved and are reported herein. This work has shown that the effective pK(2), of carboxylic acid groups in such an assembly varies linearly with the coverage of the ionized carboxylate, with a limiting value of 5.4 as this coverage approaches zero. This acid-base behavior is in agreement with that reported for other carboxylic acid bearing surfaces, as determined by a variety of other techniques.