Layer-by-layer deposition of polyelectrolytes onto poly(ethylene terephthalate) (PET) film is reported as a method for polymer surface modification. Poly(allylamine hydrochloride) (PAH) and poly(sodium styrenesulfonate) (PSS) were sequentially adsorbed to unmodified (neutral) PET as well as PET samples that had been surface modified to contain carboxylate (PET-CO2-) and ammonium (PET-NH3+) functionality. XPS and contact angle data indicate that the layers are extremely thin (2-6 Angstrom) and stratified to a significant extent. The wettability of the multilayer films is controlled by the outermost polyelectrolyte layer and the thickness of the individual layers (sublayers have an influence in multilayers with thinner layers). The substrate surface chemistry controls the thickness of all of the layers in the multilayer assembly as well as the stoichiometry (ammonium ion:sulfonate ion ratio) of the deposition process. The effect of the ionic strength of the polyelectrolyte solutions was studied for the case of the neutral substrate (PET); both the individual layer thickness and deposition stoichiometry are affected by ionic strength. Peel tests indicate that the multilayer assemblies have significant mechanical strength.