An apparatus designed to completely vaporize an intake of heavy hydrocarbon feedstock is described. The apparatus, a so-called heavy feed mixer, is comprised of pipes being disposed coaxially about a common longitudinal axis. The inner tubular section delivers a two-phase liquid-vapor mixture of hydrocarbon feedstock and dilution steam to the apparatus. The converging/diverging tubular section has a unique structure which converges to a throat section and then diverges to an outlet section. The converging section directs a uniform shroud of superheated steam onto the hydrocarbon stream delivered by the inner tubular section. Impingement of the superheated steam with the intake stream initiates mixing and further vaporization within the throat section. The mixture traverses the converging/diverging tubular section and passes into the outlet section where vaporization is complete. The completely vaporized stream is directed out of the apparatus for further processing downstream.