We investigate static electrification due to asymmetric rubbing. Eight different plastic materials, for which self-consistent electrostatic series can be obtained, are investigated here. Two rotating disc samples whose edges brought into contact are rubbed each other. The surface voltages of the rubbed surfaces are measured systematically changing the degree of asymmetry of the rubbing by varying the rotation speeds of the two discs. It is found that the tendency of the charge separation depends on the species of materials. This might indicate there are two or more mechanism of charge separation. (C) 2004 Elsevier B.V. All rights reserved.