The aim is to refine a method of establishing a predetermined viscosity when recycling polyester wastes having inherently different viscosities in such a way that the desired final material has the appropriate viscosity after a fairly short time, and with little energy consumption, and the color of the melt is also to be influenceable. To this end, it is proposed that the polyester wastes are melted in a first zone (9) of an extruder (1) and the polyester wastes, due to the moisture introduced with them, undergo hydrolytic degradation, in a second zone (11) of the extruder (1) polycondensation takes place and hydrolytic degradation is counteracted and equalization of the viscosity differences is started, in a third zone (13) of the extruder (1) a solvent such as water or alcohol is added such that hydrolytic and/or glycolytic degradation of the melt take(s) place and result(s) in uniform viscosity of the melt, in a fourth zone (14) of the extruder (1) active mixing of the melt takes place and the melt is transferred from the extruder (1) into a polycondensation reactor (6), and the final desired viscosity of the melt is set in the polycondensation reactor (6).