The phase behaviour of athabasca bitumen vacuum bottoms (ABVB) + n-dodecane + hydrogen mixtures was investigated at temperatures and pressures ranging up to 725 K and 7 MPa using an X-ray view cell. Preliminary phase diagrams are presented. The most complex phase behaviour observed is tentatively identified as "S"LLV, although the nature of the "solid" is unclear. From the slope of the "S"LV to LV phase boundary, Delta H-FUSION is less than 7 kJ/kg. Thus it may be an amorphous solid, a viscous liquid, or micellular in nature. Observed multiphase behaviour is also compared with operating conditions for well known heavy oil upgrading processes. Most processes appear to skirt multiphase zones. A preliminary four component model for the phase behaviour of the ABVB + hydrogen system has been devised using CMG-PROP and HYSIM. An example calculation comparing modelled and experimental phase behaviour is also presented.