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.