The performance of two deep eutectic solvents (DESs) was comparatively inspected in upgrading of heavy residual fuel oil (Mazut) as a chemical additive at different processing temperatures and times of 300-350 degrees C and 105-420 min, respectively. DESs consisted of ethylene glycol possessing hydrogen-donating groups and tetrapropylammonium bromide (TPABr) or choline chloride (ChCI) possessing alkyl tails with different lengths. Characterization of upgraded liquids processed at 325 degrees C for 210 min showed that TPABr:EG was three times more effective than ChCI:EG in asphaltene conversion and produced a lighter fuel with higher maltene selectivity. Besides the asphaltene reduction (36%), it was more efficient in desulfurization, with a 12.8% reduction, than dry thermal upgrading, with 5.8% asphaltene and 1.9% sulfur reductions, respectively. Moreover, upgraded oil from the TPABr:EG-incorporated scenario revealed high stability toward regression, and viscosity was well preserved after 3 months. Aside from viscosity measurement, asphaltene determination, elemental CHNS, FTIR, and TGA analyses on upgraded oils, isolated asphaltenes from TPABr:EG-incorporated and dry thermal processes were further characterized by FTIR and NMR to pursue the upgrading events. It was realized that TPABr:EG boosted the structural changes in heavy asphaltenic islands by fragmentation and dealkylation of the heteromacromolecular rings. In fact, it intensified both side aliphatic chain shortening and aromatic core shrinkage appreciably compared to the dry thermal process. On the basis of all of the findings elucidating the dual radical generation and hydrogen donation functionality of TPABr:EG, a mechanism is proposed to shed light on the pathway of influence of this eutectic mixture in the intensification of upgrading heavy oil.