Abstract: A process is described for reducing the viscosity of heavy hydrocarbon oils which comprises separately heating a stream of heavy hydrocarbon oil and a stream of gas, mixing the hot gas and hot heavy hydrocarbon oil under pressure and immediately thereafter passing the heavy oil/gas mixture through a small nozzle or orifice such that a substantial pressure drop occurs across the orifice and the heavy oil/gas mixture is ejected from the orifice as a spray in the form of fine oil droplets entrained by highly turbulent gas flow. This spray is discharged into a confined reaction zone from which the oil of reduced viscosity is collected.