Polyethylene terephtalate (PET) is a thermoplastic polyester with outstanding thermal and chemical resistance properties. Due to the fact that it is not biodegradable, the increased consumption of PET led to a significant accumulation of waste into landfields. Chemical recycling allows controlled cleavage of the polymer with isolation of the monomers that can be reused in the PET manufacture or to obtain other valuable chemicals. The most studied method for PET recycling is glycolysis with ethylene glycol and/or other glycols [1]. The reaction follows a transesterification mechanism, reason why metal salts, especially heavy metals are used as catalysts. Recently, organocatalysts and especially, bifunctional organocatalysts have been investigated and turned out to be a good choice for improving the PET destruction rate [2]. Because of their acid-base character, they activate both, the ester carbonyl group through the nucleophile component and the reactants (alcohols or amines) through their acidic part. Based on our expertise in glycolysis of PET wastes involving biomass alcohols [3], we have modeled the transesterification reaction between dimethyl terephthalate (DMT) and isosorbide (IS) in the presence of 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD). Our goal was to determine whether the dual function of the catalysts is responsible for the occurrence of the trans-esterification reaction. In this case, two alternatives were investigated: i) the formation of the hydrogen bonds between the catalyst and the diol and ii) the acyl transfer with the formation of the amide intermediate. For this purpose, quantum chemical calculations using DFT method with a B3LYP functional and a TZVP basis set were used to investigate the reaction mechanism. These theoretical calculations were corroborated with 1H-NMR spectroscopy in solution. If methanol is present in the reaction medium, the first pathway is more likely to be followed whereas, if methanol is removed from the reaction medium, the dual function of the guanidinic catalyst is responsible for the catalytic activity.schemeThis work was supported by UEFISCDI grant PNII-PT-PCCA-2013-4-1388, no61/2014PERCIT.