A process for catalytically cracking a hydrocarbon oil containing sulfur and/or nitrogen hydrocarbon constituents by dissolving excess hydrogen in the liquid hydrocarbon feedstock in a mixing zone at a temperature of 420° C. to 500° C. and a hydrogen-to-feedstock oil volumetric ratio of 300:1 to 3000:1, flashing the mixture to remove remaining hydrogen and any light components in the feed, introducing the hydrogen saturated hydrocarbon feed into an FCC reactor for contact with a catalyst suspension in a riser or downflow reactor to produce lower boiling hydrocarbon components which can be more efficiently and economically separated into lower molecular weight hydrocarbon products, hydrogen sulfide and ammonia gas and unreacted hydrogen in a separation zone. Hydrogen present in the liquid phase enhances the desulfurization and denitrification reactions which occur during the conversion process and allows for the removal of significantly more sulfur- and/or nitrogen-containing contaminants from the feedstock in an economical fashion.