Abstract: A pretreatment process is described for heavy hydrocarbon oil feedstock, such as oils extracted from tar sands. The feedstock is passed through a heated, continuous flow electron or x-ray treatment zone. The process is designed to allow the feedstock to be conditioned with ozone-containing air, steam or a hydrogen donor gas prior to electron/x-ray treatment. The ozone-containing air stream may be the stream produced in the electron treatment zone. After conditioning, the heavy oil is heated to a specified temperature and uniformly treated with high-energy beams of electrons or x-rays. A key feature of the invention is the electron/x-ray treatment zone may use multiple accelerators or a beam splitter to ensure acceptable dose distributions in the flowing feedstock. Another key feature is the recirculation of volatiles back into the feedstock.