A method for selectively separating and simultaneously increasing the purity of two fragmented used synthetic organic materials, having distinct densities, mixed together and mixed with various contaminating materials to be eliminated, having a density lower or higher than 1, using dense aqueous media, with each aqueous medium having a selected density "ds." as the separation threshold of said organic materials and contaminants, with said separation resulting in two flows, a supernatant one and another settling one, with said method being characterized in that it comprises: a) A step of high density separation ds(a) of the mixture into two flows, a supernatant one and another settling one using an aqueous medium the density ds(a) of which is so selected as to be intermediate relative to the densities of one and the other synthetic organic materials to be separated, preferably close to the density of the organic material to be separated with the lowest density so as to ensure the complete flotation thereof, b) A step of low density separation ds (b) of the compounds of the supernatant flow using an aqueous medium, the density ds(b) of which is so selected as to be lower than ds(a) in order to eliminate a supernatant flow consisting of contaminants and to collect a settling flow consisting of the fragments of the first used synthetic organic material to be recycled, c) A step of high density separation ds(c) which is greater than ds(a) of the compounds of the settling flow using an aqueous medium, the density ds(c) of which is so selected as to be at least equal to the density of the synthetic organic material present in said flow, with a supernatant flow (d) consisting of the mixture of the fragments of the synthetic organic material with a density ds(c) and contaminating materials having densities between ds(a) and ds(c), and with a settling flow consisting of the contaminating materials having a density lower than ds(c) which is eliminated, d) A step of low density separation ds (d) of the supernatant flow (d) using an aqueous medium, the density ds (d) of which is so selected as to be lower than ds(c), the supernatant flow of which consists of the contaminating materials having densities between ds(a) and ds(c), and is eliminated, and the settling flow of which, consisting of the fragments of the homogeneous used synthetic organic material, having a density ds(d), is collected to be recovered. a) A step of high density separation ds(a) of the mixture into two flows, a supernatant one and another settling one using an aqueous medium the density ds(a) of which is so selected as to be intermediate relative to the densities of one and the other synthetic organic materials to be separated, preferably close to the density of the organic material to be separated with the lowest density so as to ensure the complete flotation thereof, b) A step of low density separation ds (b) of the compounds of the supernatant flow using an aqueous medium, the density ds(b) of which is so selected as to be lower than ds(a) in order to eliminate a supernatant flow consisting of contaminants and to collect a settling flow consisting of the fragments of the first used synthetic organic material to be recycled, c) A step of high density separation ds(c) which is greater than ds(a) of the compounds of the settling flow using an aqueous medium, the density ds(c) of which is so selected as to be at least equal to the density of the synthetic organic material present in said flow, with a supernatant flow (d) consisting of the mixture of the fragments of the synthetic organic material with a density ds(c) and contaminating materials having densities between ds(a) and ds(c), and with a settling flow consisting of the contaminating materials having a density lower than ds(c) which is eliminated, d) A step of low density separation ds (d) of the supernatant flow (d) using an aqueous medium, the density ds (d) of which is so selected as to be lower than ds(c), the supernatant flow of which consists of the contaminating materials having densities between ds(a) and ds(c), and is eliminated, and the settling flow of which, consisting of the fragments of the homogeneous used synthetic organic material, having a density ds(d), is collected to be recovered.