Using an interfacial force microscope (IFM), we have measured the elastic nanoindentation modulus of thin films of poly[(aminopropyl)siloxane] spin-coated on Nai-containing glass and native SiO2. The films were prepared by the hydrolytic polycondensation of gamma-(aminopropyl)triethoxysilane (gamma -APS). The Elastic moduli of 500 Angstrom thick films deposited on Naf-containing glass and native SiO2 are 8 +/- 2 and 35 +/-3 G Pa, respectively. IFM data were complemented with atomic force microscopy and infrared and X-ray photoelectron spectroscopy. We propose that the significantly smaller modulus of gamma -APS on glass is related to the incorporation of Na+ ions from the glass into the siloxane network of the film. These incorporated Na+ ions act as Lewis acids and catalyze the depolymerization of the Si-O-Si network, resulting in a less rigid polysiloxane framework and a lower elastic modulus. A clue to the films' structural and chemical difference is provided by the observation of time-dependent morphological changes for gamma -APS on glass but not for gamma -APS on SiO2.