A process for in-situ upgrading of a heavy hydrocarbon includes positioning a well in a reservoir containing a heavy hydrocarbon having an initial API gravity no greater than about 20, an n-heptane asphaltene content at least about 1 wt. %, and a viscosity at 35° C. greater than about 350 centistokes; injecting hydrocarbon solvent(s) and asphaltene precipitant additive(s) into the well at a ratio by volume of the solvent to the heavy hydrocarbon of about 0.1:1 to about 20:1 under reservoir conditions so as to provide an upgraded hydrocarbon in the reservoir having an improved API gravity, a reduced asphaltene content, and a lower viscosity; and producing the upgraded hydrocarbon from the well. The process of the present invention can be also carried out at higher temperatures such as by injecting steam with the one or more hydrocarbon solvents and the one or more asphaltene precipitant additives into the well.