The solvent-assisted bitumen recovery processes, such as vapor extraction (VAPEX) and expanding-solvent steam-assisted gravity drainage (ES-SAGD), are designed on the basis of the drainage of diluted oil at in situ conditions. Thus, the density of oil and its diluted mixtures is required to predict the production rate and cumulative oil recovery. In this study, the density of Athabasca bitumen and its mixtures with hexane at different mass fractions (0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) has been accurately measured. The experiments were taken at temperatures changing from ambient up to 463 K and at pressures up to 10 MPa. The volume change upon mixing for the mixtures was investigated from the experimental results, and the influence of pressure, temperature, and solvent mass fraction on the volume change upon mixing and density was studied. The density data are evaluated by no volume change, excess volume, and effective liquid density calculation methods. It was found that the mixture data are well-predicted using effective liquid densities with an average absolute relative deviation (AARD) of 0.31%.