Thin films of crosslinked polymethylhydrosiloxane (PMHS) have been grafted on silica using the sol-gel process allowing further functionalization by effective quantitative hydrosilylation of SiH groups by olefins within the network. Post-functionalization gives the polysiloxane network with n-alkyl side chains. The PMHS coating was prepared by room temperature polycondensation of a mixture of methyl-diethoxysilane HSiMe(OEt) 2 monomer and triethoxysilane HSi(OEt) 3 (TH) as cross-linker. The surface-attached films are chemically stable and covalently bonded to the silica surface. Subsequently, films were functionalized without delamination. We showed by FTIR spectroscopy how the crosslinking ratio and the molecular size of the alkenes precursors influence the extent of the hydrosilylation reaction of SiH groups in the PMHS network. We have determined that quasi-full olefin addition cat-alyzed by a platinum complex occurred within soft networks of less than 5% TH with 1-alkenes CH 2 ¼ ¼CH(CH 2) n-2 CH 3 of various alkyl chain lengths (n ¼ 5, 11, 17). Powders of PMHS gel were also modified with 1-alkenes by hydrosilylation. The SiH groups within the soft gel (5% crosslinked) were fully functionalized as shown by 29 Si and 1 H solid-state NMR. The structure of functionalized polysiloxane with n-octa-decyl and n-dodecyl side chains was studied by FTIR, wide angle X-ray diffraction, and DSC showing crystallization of the long n-alkyl chains in the network.