The injection of hydrogen or hydrogen-carbon dioxide mixtures provides an additional method for the recovery of heavy oils. By attacking oxygenated and sulfided cross linkages in the molecular structure of the oil, a degree of upgrading is effected, producing flow mobility over the long term. The residence time necessary may be measured in days, weeks, months, or even years. Hydrogen and carbon dioxide can be produced most readily from the (catalytic) steam gasification of coal or other carbonaceous materials and, in particular, of part of the heavy crude itself. The economics are such that costs will be only minor in terms of the net crude oil produced.