We describe computational investigations on mechanisms for the organocatalyzed trans-esterification and amidation of poly(ethylene) terephthalate (PET). These processes were modeled by the reactions of methylbenzoate (MB) with ethylene glycol (EG), ethylenediamine (EDA) and ethanolamine (EA) organocatalyzed by 1,5,7-triazabicyclododecene (TBD). Mechanistic pathways for reactions in which TBD acts as the lone catalyst have been compared with mechanisms in which an additional molecule of the amine or alcohol acts as a cocatalyst. Calculations suggest that the combination of an organocatalyst with a molecule of an alcohol or amine cocatalyst is typically more activating than a lone catalyst. Our results demonstrate that alcohols are more activated than amines during nucleophilic attack. However, while transesterification may be faster than amidation, this process may be reversible, in contrast with amidation which leads to stable products.