A process for treating a fiberglass-reinforced plastic waste comprising: - a matrix of an unsaturated polyester resin; - reinforcement glass fibers incorporated in said matrix, said process comprising the steps of: - prearranging a reactor comprising a pyrolysis chamber; - feeding said fiberglass-reinforced plastic waste into said pyrolysis chamber; - removing oxygen from said pyrolysis chamber down to a predetermined residual oxygen concentration (C R); - creating a CO 2 -containing environment in said pyrolysis chamber, at a 30% minimum CO 2 volume concentration; - heating said fiberglass-reinforced plastic waste in said pyrolysis chamber and reaching a pyrolysis temperature (T P) set between 350°C and 550°C; wherein said step of removing oxygen and said step of reaching a pyrolysis temperature (T P) are carried out in such a way that said residual oxygen concentration (C R) is attained before reaching said pyrolysis temperature (T P); - maintaining said CO 2 -containing environment and said pyrolysis temperature (T P) in said pyrolysis chamber for a predetermined residence time, obtaining - a gas mixture containing products of pyrolysis of said unsaturated polyester resin, and - a solid residue comprising said glass fibers; - extracting said gas mixture from said pyrolysis chamber; - condensing said pyrolysis products from said gas mixture in a condensation chamber obtaining a condensate liquid phase, comprising a main amount of the product of pyrolysis that it is separated from the uncondensed gas, such that said organic liquid can be recycled as such to a production process of fiberglass-reinforced plastics as a mixture with commercial unsaturated polyester resins; - extracting said solid residue from said pyrolysis chamber. - a matrix of an unsaturated polyester resin; - reinforcement glass fibers incorporated in said matrix, - prearranging a reactor comprising a pyrolysis chamber; - feeding said fiberglass-reinforced plastic waste into said pyrolysis chamber; - removing oxygen from said pyrolysis chamber down to a predetermined residual oxygen concentration (C R); - creating a CO 2 -containing environment in said pyrolysis chamber, at a 30% minimum CO 2 volume concentration; - heating said fiberglass-reinforced plastic waste in said pyrolysis chamber and reaching a pyrolysis temperature (T P) set between 350°C and 550°C; wherein said step of removing oxygen and said step of reaching a pyrolysis temperature (T P) are carried out in such a way that said residual oxygen concentration (C R) is attained before reaching said pyrolysis temperature (T P); - maintaining said CO 2 -containing environment and said pyrolysis temperature (T P) in said pyrolysis chamber for a predetermined residence time, obtaining - a gas mixture containing products of pyrolysis of said unsaturated polyester resin, and - a solid residue comprising said glass fibers; - a gas mixture containing products of pyrolysis of said unsaturated polyester resin, and - a solid residue comprising said glass fibers; - extracting said gas mixture from said pyrolysis chamber; - condensing said pyrolysis products from said gas mixture in a condensation chamber obtaining a condensate liquid phase, comprising a main amount of the product of pyrolysis that it is separated from the uncondensed gas, such that said organic liquid can be recycled as such to a production process of fiberglass-reinforced plastics as a mixture with commercial unsaturated polyester resins; - extracting said solid residue from said pyrolysis chamber.