Abstract An extra-heavy crude oil underground upgrading process is described which involves the downhole addition of a hydrogen donor additive (tetralin) under steam injection conditions. Using a batch laboratory reactor or a continuous bench scale plant (280-315°C and residence times between 24-64 h), physical simulation experiments showed an increase of at least 3° in API gravity of the treated Extra-Heavy Crude Oil, three-fold viscosity reduction and, approximately, 8% decrease in the asphaltene content with respect to the original crude. It was found that the presence of the natural formation (catalyst) and methane (natural gas) is necessary to enhance the properties of the upgraded crude oil. Compositional-thermal numerical simulations were carried out and the results showed a good match between the calculated and experimental °API gravities of the upgraded crude oil (average relative error 1-4%) for all conditions studied. Similar results were obtained with the asphaltene contents (14-23%), percentage of conversion of the >500°C fraction (12%) and tetralin (16-23%).