The Effect of Molecular Orientation on the Mechanical Properties of Polystyrene presents the results of a study that investigated the effect of molecular orientation on the mechanical properties of polystyrene. Oriented sheets of two commercial homopolystyrenes (PSA and PSB) differing in molecular weight, rheological and glass transition temperature properties were prepared by sheet extrusion and drawing. The properties of the resulting sheets were examined at seven different laboratories in Europe. The study focused on length reversion ratio, birefringence, orientation stress, shrinkage stress, and various mechanical properties such as rupture stress and strain, the specific energy of rupture, and sonic modulus and relaxation modulus. Impact behavior and environmental stress cracking behavior were also evaluated. One of the findings was that oriented polymers subjected to lateral strains can develop catastrophic crack growth when immersed in an active liquid. The onset of the cracks depends on the lateral strain imposed as well as the degree of orientation. PSB was more resistant than PSA. This monograph should be a useful resource for chemists.