The dynamic-mechanical behaviour and the tensile moduli of unstretched and stretched semicrystalline s-PS films, presenting different polymorphic forms (alpha, gamma, delta and clathrate) but similar Crystallinity and orientation, have been compared. The main aim is to elucidate the possible influence of different crystalline phases, being largely different in chain conformation and density, on mechanical properties of s-PS semicrystalline samples, For unstretched films presenting a preferential perpendicular orientation of the chain axes, the highest elastic modulus is observed for films with the high density gamma phase while for uniaxially oriented films the highest modulus is observed for films with the trans-planar alpha phase. As for the clathrate films, the guest molecules when only included into the crystalline clathrate phase, have no plasticizing effect. (c) 2005 Elsevier Ltd. All rights reserved.