Abstract: Disclosed is a process for the preparation of polyesters, comprising the following steps and characterized by the following measures: a) suspending polyethylene terephthalate waste in ethylene glycol and/or in 1,2-propylene glycol containing a transesterification catalyst, b) heating the reaction mixture to depolymerize the polyethylene terephthalate waste for a time sufficient to decompose the polyethylene terephthalate into monomers and into oligomers, c) addition of end group forming monomers in the form of polyalkylene glycol monoalkyl ethers, fatty alcohols, fatty amines, fatty acids or esters thereof or mixtures of two or more thereof, and/or sulfo(poly)alkylene glycols and/or sulfoaryl carboxylic acids or esters thereof, d) addition of sulfoarylene dicarboxylic acids or polyester-forming derivatives thereof, e) optionally adding aliphatic diols, cycloaliphatic diols, organic dicarboxylic acids, their polyester-forming derivatives, crosslinkers or mixtures of two or more thereof, f) heating the react