Crosslinked polymethylhydrosiloxane (PMHS) thin films prepared by sol-gel polycondensation have been functionalized by Pt-catalyzed hydrosilylation of SiH groups with an allyl ureido crown ether precursor. To this purpose, both 4 0-allylurea-benzo-15-crown-5 (1) and 1-allyl-3-propyl-urea (2) were synthesized and characterized. We have shown that competitive side-reactions occurred following hydrosilylation due to the hydrolysis of part of the SiH groups resulting in the formation of new crosslinks Si(CH 3)O 3/2 as shown by solid-state 29 Si-NMR. This is explained by the deactivation of the Pt catalyst toward hydrosilylation by amide groups. For thin films ($ 1 lm) prepared on silicon wafers, a quantitative method based on FT-IR transmission spec-troscopy was used to measure the crosslinking density of the network, and the percentage of functionalization (SiC %) following hydrosilylation. The results are discussed in relation to the mesh size of the network, and the diffusion of alkenes and water molecules within lightly crosslinked PMHS gels obtained by varying the amount of triethoxysilane crosslinker (mol %) from 15 to 1%. The self-organization properties of ureido groups by H-bonding were studied by FT-IR for the functionalized thin films. The complexation properties of the crown ether 1-functionalized thin films were evidenced by using FT-IR following diffusion-reactions of NaSCN and KSCN salts in CHCl 3 : MeOH solvent mixtures within thin films.