In this thesis, an accurate partial phase diagram for the model heavy oil system athabasca bitumen vacuum bottoms (ABVB) (24.6 wt. %/2 mole %) + dodecane (73.8 wt. %/47 mole %) + hydrogen (1.6 wt %/51 mole %) was constructed in the temperature range 425 K to 725 K and the pressure range 2 MPa to 7 MPa using a novel x-ray view cell apparatus. This model heavy oil fluid system is shown to exhibit L₁V and L₁L₂V phase behaviour over parts of this P-T region. The phase boundaries separating these regions are shown to be reversible within 2 K and 0.03 MPa. The shape of the low temperature boundary between the L₁V and L₁L₂V zones is characteristic of dilute asymmetric mixtures where on isothermal compression a heavy liquid phase, L₂, appears then disappears within the light liquid phase, L₁. Such phase behaviour is referred to as unusual retrograde condensation and is of both practical and theoretical interest. For this mixture the retrograde condensation extends over a 75 K and 2 MPa temperature and pressure range, respectively. This is only the second reported large scale example of such phase behaviour.

Of equal importance is the finding that irreversible phase transitions involving asphaltene precipitation arise within the L₁L₂V region and not in the L₁V region--even at higher temperatures. Transitions between the multiphase regions were found to be reversible at temperatures less than 655 K, in all cases. At higher temperatures irreversible "asphaltene precipitation" arose within the L₂ phase. "Asphaltene precipitation" did not arise in the absence of the L₂ phase, i.e.:​ within the L₁V region, even at temperatures in excess of 700 K. These data provide a strong link between "asphaltene precipitation" and multiphase behaviour, and demonstrate a physical rather than a purely kinetic basis for asphaltene precipitation at elevated temperatures. This result compels one to rethink the approach to the modelling of such systems from both a phase behaviour and reaction kinetics perspective.

A third important result for this work is the development of a variable volume view cell suitable for studying the phase behaviour of opaque hydrocarbon systems and other systems. This device, created in collaboration with D. B. Robinson Ltd., greatly simplifies phase behaviour experiments and allows one to work with small volumes of fluid over a broader range of temperatures and pressures (0.1-27.5 MPa, 290-750 K). The apparatus is being considered for commercialization.