The heaviest fraction of crude oils, asphaltenes, has been shown to play a central role in the stabilization of troublesome water-in-crude oil emulsions. In previous model oil systems, emulsion stability was observed to depend notably on the properties of self-assembled interfacially active asphaltenic aggregates. Thus, careful characterization of these aggregates is of great importance for better understanding of asphaltene-stabilized emulsions. Small-angle neutron scattering (SANS) has proven to be a powerful tool for such characterization. Described here is the application of SANS on asphaltenes from Hondo crude oil dissolved in binary solvent mixtures of toluene with n-heptane, decalin, or 1-methylnaphthalene. A polydisperse oblate cylinder (POC) form factor model was used to fit the aggregate scattering data, and subsequent calculations of minimum error were performed to ascertain the entrained solvent composition within the aggregates. When toluene was paired with either a weak solvent (decalin) or a precipitant (n-heptane), the entrained solvent composition was nearly pure toluene (> 90%). Conversely, for the toluene/1-methlnaphthalene pairing, methylnaphthalene demonstrated slight preferential entrainment (similar to 60%) compared with the bulk composition (similar to 57%). This is the first such in situ experimental determination of the aggregate composition for asphaltenes dissolved in binary solvents, reinforcing the important role of asphaltene-solvent interactions in asphaltene self-assembly.