Chemical stabilization of air-unstable molecules, by their inclusion as guest of nanoporous-crystalline phases of syndiotactic polystyrene (s-PS), is for the first time revealed. FTIR analyses show that antimicrobial hexanal molecules are chemically stable for months when guest of s-PS cocrystalline phases, while they easily oxidize to hexanoic acid when they are simply dissolved in polymer amorphous phases. Formation of s-PS/hexanal cocrystalline forms is clearly demonstrated by polarized FTIR spectra and by X-ray diffraction patterns. Chemical stabilization is observed for hexanal molecules being guest of both nanoporous-crystalline forms of s-PS, i.e., for guest molecules isolated in the cavities of d form or aligned along the channels of e form. Films exhibiting s-PS/hexanal clathrate forms are able not only to chemically stabilize the antimicrobial molecules but also to give their long-term release (with a diffusivity close to 2 x 10(-14) cm(2) s(-1)). Possible mechanisms of chemical stabilization by clathrate formation are discussed.