Small-angle X-ray scattering (SAXS) analyses of an asphaltene (a heptane-insoluble fraction in Canadian oil sand bitumen (CaAs)) at various concentrations in bromobenzene (BB) were performed at a synchrotron facility. BB is the first trial medium in which the aggregation behavior of asphaltenes has been elucidated, and is considered to be one of the "best" pure solvents for CaAs when determining the Hansen solubility parameters (HSP). Although the aggregation behavior of the CaAs in toluene (TL) and toluene-pentane mixed solvent (TL-PT10, containing 10% pentane on a volume basis) was confirmed to be similar to that reported in previous S.AXS studies, the behavior in BB was markedly different. The results indicated that aggregates with a soft boundary of similar to 30-60 angstrom in the radius of gyration (R-g), which were observable in TL and TL-PT10, disappeared in BB and larger aggregates with a clear boundary appeared simultaneously. This phenomenon supported a colloidal aggregation model, with HSP analyses suggesting that BB dispersed the colloid surface fraction at the molecular level and isolated the colloid core fraction, which led to the formation of a rigid aggregation of the core fraction. The HSP analyses enabled us to evaluate the aggregation behavior quantitatively, and the results obtained by SAXS were consistent with those obtained by Rayleigh scattering that we reported previously.