the invention relates to a method for selectively separating mixed synthetic organic materials such as filled or non - filled polymers and / or copolymers that are wastes , particularly used , for recycling in order to upgrade them . these synthetic organic materials result from the destruction by crushing of automobiles and durable consumer goods that have reached the end of their serviceable lives . said selective separation method acts by separating these materials with regard to a density threshold selected in a dense medium consisting of separating fluid liquid suspensions composed of powdery particles dispersed in an aqueous phase . these suspensions are stabilized by using a dynamic stabilizing means at a density threshold value selected for causing the selective separation of a determined fraction from the mixture of the used materials to be separated .