Three different co-crystalline phases of syndiotactic polystyrene (s-PS) with azobenzene have been prepared and structurally characterized. The triclinic delta-clathrate phase is characterized by a styrene-monomeric-unit/guest ratio (SU/G) equal to 8/1 while the delta-intercalate phase is characterized by SU/1G = 4/1. The orthorhombic epsilon-clathrate phase allows stacking of a variable number of guest molecules per monomer unit with SU/G >= 9/1. Isomerization reactions of the azobenzene guest molecules (both photo-induced trans -> cis and spontaneous cis -> trans) have been studied by UV-Visible and FTIR measurements. Repeated isomerization cycles produce progressive desorption from the polymer film of azobenzene molecules being simply dissolved in s-PS amorphous phases, while poorly affect the content of azobenzene molecules being guest of s-PS co-crystalline phases. Polarized FTIR spectra of axially oriented films indicate that isomerizations occur without expulsion of azobenzene guest molecules from the polymer co-crystalline phases. The whole set of data indicate that films exhibiting co-crystalline phases of azobenzene with s-PS are suitable for producing self-erasing photo-isomerization patterns with molecular size marks. (c) 2012 Elsevier Ltd. All rights reserved.