Athabasca bitumen and other denser-than-water reservoir fluids are coproduced with water or are produced using water or steam injection. Organic solvent injection into reservoirs to reduce hydrocarbon resource viscosity and hence improve production rates is envisaged. A consequence of solvent addition is that the density of the resulting hydrocarbon-rich phase can drop below that of the water-rich phase over broad ranges of composition, temperature, and pressure. In this contribution, densities of Athabasca bitumen from 20 to 200 degrees C at 0.10, 1.12 and 3.29 MPa, Athabasca bitumen + heptane from 10 to 80 degrees C at 0.10 MPa and 30 to 140 degrees C at 0.10, 1.11, and 3.29 MPa and Athabasca bitumen + toluene from 10 to 80 degrees C at 0.10 MPa were measured (using an Anton Paar DMA 5000 and an Anton Paar DMA HP) and compared with those of pure and produced liquid water from 20 to 200 degrees C at 0.10, 1.12, and 3.29 MPa. Phase order inversion is shown to overlap with envisaged processing conditions for production, transport, and refining of denser-than-water reservoir fluids because only low mass fractions of solvent are required to cause inversion to occur. The phase order inversion envelopes also include compositions that invert twice as a function of temperature, and the upper temperature associated with phase inversion is a function of pressure. Remote from the critical point of added solvents, the phase inversion boundary is shown to be well-approximated by the ideal mixing assumption applied to Athabasca bitumen + solvent pseudobinary mixtures. The impact of solvent critical temperature on the volume of mixing for solvent + oil mixtures is discussed. Phase order inversion is a general and readily computed phenomenon.