With the merging of operating conditions for proposed high-temperature production and refining processes for heavy oil and bitumen, there is a growing interest in the phase behavior, thermophysical properties and reaction outcomes for water + heavy hydrocarbon resource mixtures at elevated temperatures and pressures. Athabasca bitumen, from Canada, is a commercially exploited hydrocarbon resource where 50 wt.% of the material possesses a nominal boiling point exceeding 797 K. In a prior publication, the phase behavior type of the Athabasca bitumen + water binary was identified experimentally as Type IIIb, according to the vanKonynenburg and Scott naming scheme. The present contribution focuses on the volume of mixing and the solubility of water in the Athabasca bitumen-rich liquid phase over the temperature interval 523-644 K and over the pressure interval 3.9-26.2 MPa. The measurements were obtained using a high-pressure variable-volume beryllium-walled view-cell, equipped with non-intrusive X-ray transmission tomography. X-ray tomography permits visual observation of the number of coexisting phases, and their corresponding volumes. Phase volume and density measurements were validated by reproducing published values for 1-methyl naphthalene up to 573.8 K. Water solubility values were validated by reproducing data for the 1-methyl naphthalene + water and toluene + water binaries up to 573.2 K and 553.2 K, respectively. Measured solubility and excess volume values are also placed in a broader context by comparison with pertinent literature data. (C) 2013 Elsevier B.V. All rights reserved.