The huge energy required to overcome the high-pressure drop in pipelines due to the high viscosity of heavy crude oils has motivated the development of technologies to improve the flow properties of these heavy hydrocarbons, such as friction reduction, viscosity reduction, and in situ upgrading. It has been reported that a series of H-donors has been used to upgrade heavy oil for their transportation (low viscosity and high API gravity). Virtually any organic compound with a low oxidation potential can serve as a useful hydrogen donor. The low oxidation potential enables the transfer of hydrogen(s) from the donor to the substrate under mild reaction conditions. The choice of donor is based on the nature of the reaction, its availability, and solubility in the reaction medium.