A correlation was developed to predict the density of mixtures of heavy oil (and other petroleum liquids) and hydrocarbon solvents when the densities of each fluid in the mixture are available. Densities at atmospheric pressure and 293 K were measured for saturates and aromatics (SA) fractions from 10 native, thermo-cracked, and hydrocracked heavy oils all mixed with toluene and n-heptane; distillation cuts from 6 heavy oils mixed with toluene; and mixtures of deasphalted heavy oils with naphtha, diesel, and condensate. Density of mixtures of hydrocarbons and solvents at higher pressures (0.1-10 MPa). and temperatures (298-353 K) were also measured or obtained from the literature. Symmetry versus mass fraction was observed for all of the mixtures, and their densities were fitted with a mixing rule in which excess volumes are quantified with a binary interaction parameter and the density of each mixture component. The excess volume mixing rule fit the data for each mixture with average absolute deviations (AAD) less than 1.1 kg/m(3), and the overall average AAD was 0.39 kg/m(3). The binary interaction parameter was correlated to the density of the components in the mixture and to temperature. Pressure was found to have no consistent effect in the interaction parameter and was neglected. The overall AAD for the density determined with the torrelated fin for binary mixtures was L. kg/m(3) compared with 3.0 kg/m(3) if regular solution behavior was assumed and 3.6 kg/m(3) when the standard American Petroleum Institute (API) correlation was used to predict the density of the mixtures. The API correlation and correlated excess volume mixing rule performed similarly for hydrocarbons with carbon numbers above five. The proposed correlation was also tested on ternary data from the literature with comparable results.