Density data are reported for the following mixtures at temperatures from 20 to 175 degrees C and pressures up to 40 MPa: ethane + n-decane, propane + n-decane, butane + n-decane, propane + toluene, and propane + cyclooctane, as well as bitumen diluted with ethane, propane, n-butane, n-pentane, n-heptane, and carbon dioxide. A straightforward excess volume based mixing rule is proposed to determine the density of liquid mixtures of hydrocarbons. Excess volumes are accounted for with a binary interaction parameter, and a correlation is proposed to estimate the interaction parameter when mixture data are unavailable. For dissolved gaseous solvents and liquid solvents near their critical point, the input to the mixing rule is their effective liquid density. Effective density correlations were developed for n-alkanes from methane to n-heptane and carbon dioxide. The method is only valid for mixtures in the liquid region with a reduced temperature below approximately 0.52 (or higher at higher pressure). The mixing rule with no excess volume predicted the density of over 60 binary mixtures of liquid hydrocarbons at 25 degrees C and hydrocarbon mixtures with dissolved gas components over a broad range of pressures and temperatures with an average deviation of less than 1% when the criterion for validity was met. The density of diluted bitumen mixtures was also predicted with the same accuracy. All of the data were fitted to within experimental error when using binary interaction parameters.