This paper presents a study and some considerations regarding the electrical tribocharge and electrostatic separation of plastic materials (polymers) using the triboelectric effect. The dependence of the electrical tribocharge on the relative humidity of the atmosphere in which the process takes place were analyzed, based on a theoretical model regarding the charge transfer between two dielectric surfaces. The charge transfer takes place through an intermediate water layer interposed between surfaces belonging to different polymer particles with different work functions. For this purpose a mixture containing two types of plastic materials (in this case the polymers polyethylene and polystyrene) was separated using a free-fall separation system. The particles are separately collected as a result of their electrical properties. Finally, the results and comments regarding the obtained vales for grade and recovery are given. (C) 2003 Elsevier Ltd. All rights reserved.