A monoclinic -clathrate form of syndiotactic polystyrene (s-PS) with carvacrol (a relevant natural phenolic antimicrobial) has been prepared and characterized by X-ray diffraction. Very informative are Fourier transform infrared spectra, in particular their OH stretching region that shows a narrow peak and a broad band, corresponding to carvacrol molecules being isolated guest of the co-crystalline phase or dissolved in the amorphous phase, respectively. Analogous spectral features allow discriminating, for many different s-PS guests, between molecules being in crystalline or in amorphous phases. s-PS co-crystalline films with carvacrol molecules being prevailingly (more than 90%) guest of the co-crystalline phase have been prepared, even for high carvacrol content (up to 10-11 wt %). The location of most antimicrobial molecules in the crystalline phase assures a decrease of desorption diffusivity of two to three orders and hence long-term antimicrobial release. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 657-665