X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) measurements were made on asphaltenes and vacuum residua (denoted by the prefixes As and VR, respectively) isolated from three different crude oils-Maya (MY), Khafji (KF), and Iranian Light (IL)-to characterize the petroleum asphaltene aggregates present under various conditions. In the XRD experiments, the samples were loaded on a small plate sample holder that was kept horizontal while measurements were made at 30, 150, and 300 degreesC. The layer distances between aromatic sheets of asphaltenes were similar to3.6 Angstrom, and the number of aromatic sheets in a stacked cluster decreased from eight to five as the temperature increased from 30 degreesC to 300 degreesC. The different crystallite parameters varied little between the three asphaltenes, although maltenes in the vacuum residua interacted with the asphaltenes and loosened their stacking by a small amount. In SAXS experiments, scattering patterns were obtained on the dry asphaltenes at room temperature in a flowing nitrogen atmosphere and the samples were then heated from 30 degreesC to 500 degreesC. The fractal aggregates of As-MY, As-KF, and As-IL broke down at 241, 179, and 243 degreesC, respectively. From these results, and earlier small-angle neutron scattering (SANS) data, a hypothetical hierarchical model of asphaltene aggregation is proposed.