Abstract: A method to work-up waste material of polyethylene terephthalate (PET) in solid form and having a high initial viscosity for multivalent reuse and to recondense it to a high viscosity. The method includes the chemical regeneration and conversion of the recyclate in a mixture with a primary component in continuous and batchwise systems to form a homogenous product. The material to be reprocessed is heated in a stream of gas, crystallized, and dried. The solid-phase condensation is concluded in a stream of gas at high temperature. A transformation process is carried out by mixing the recycling material after treatment with a second component containing hydroxyl end groups.