Feed vaporization in the fluid catalytic cracking (FCC) process affects the yield of valuable products in risers and downers. Feedstock droplets sprayed into a fluid cracker undergo both homogeneous vaporization in the gas phase and heterogeneous vaporization as they collide with catalyst particles. This paper models both processes. Homogeneous vaporization, with both convection and radiation heat transfer, cannot completely vaporize oil droplets with diameters larger than 10 mum. Spraying droplets into a dilute cloud of catalyst particles forms a "tunnel" of hydrocarbon vapor that minimizes droplet-catalyst contact and vaporization. Alternately, spraying the droplets onto a dense jet of catalyst particles does not provide proper heat transfer and vaporization. Modeling indicated that the best way of ensuring fast vaporization of oil droplets in FCC risers and downers is to spray the droplets onto a jet of catalyst particles with a porosity ranging between 70 and 90%.