The mechanism of the discontinuous swelling/deswelling transitions exhibited by polyelectrolyte multilayers containing poly(allylamine hydrochloride) (PAH) was examined by FT-IR spectroscopy, in-situ atomic force microscopy (AFM), and in-situ ellipsometry. Assembly pH was found to play a critical role in determining the postassembly pH-dependent swelling behavior of multilayers containing PAIT Multilayer films assembled at pH < 8.5 were found to exhibit pH-independent swelling behavior over the pH range of 2.0-10.5, whereas dramatic discontinuous swelling transitions were observed when the assembly pH was greater than 8.5. FT-IR spectroscopy was used to demonstrate unequivocally that the pH-triggered, discontinuous swelling/deswelling transitions of PAH/sulfonated polystyrene (SPS) multilayers assembled at high pH (> 8.5) are driven by changes in the degree of ionization of free amine groups of PAH that are established during multilayer assembly. The pH trigger points of these swelling/ deswelling transitions are determined by the elimination/reestablishment of hydrophobically associated PAH chain segments. Such hydrophobic interactions are responsible for shifting the pKa of the free amine groups of PAH to unusually low values (ca. pH 4.0). The kinetics of deswelling were found to be strongly influenced by the type of polyanion assembled with PAH.