The structures of the deformation zones in polystyrene (PS), the so-called crazes, were investigated in detail by electron microscopy. Compared with the conventional transmission electron microscope (TEM) the usage of a high voltage electron microscope (HVEM) with an accelerating voltage of 1000 kV has several advantages: it renders possible the investigation of relatively thick specimens (thicknesses up to and exceeding 5 pan), the performance of in situ deformation tests, and the application of a special tensile device, producing a defined uniaxial straining state.First, general features of the crazes in PS (pressure-molded samples, solution-cast films) are described, and second, the crazes are precisely characterized by quantitatively measuring the electron micrographs of the crazes. Shape, thickness profile, minimum size, interior structure, and elongation of the crazes are discussed in detail.