High field conduction and space charge formation in low-density polyethylene and ethylene-styrene copolymers were studied by measuring DC conduction and thermal pulse current at various temperatures. Conduction current is considered to be dominated by traps due to benzene rings introduced into copolymers. At room temperature, injected electrons and holes seem to be trapped near the cathode and anode, respectively. At 50°C, electrons are considered to be transported by overcoming the effect of traps, and holes seem to become mobile at 90°C. The number of mobile carriers is a function of styrene content and temperature.