Small-angle neutron scattering (SANS) is used to characterize the aggregation of pure vanadyl petroporphyrins (VOPPs), as well as of asphaltenes mixed with either VOPPs or synthetic porphyrins. Simple VOPPs generally form small nanoaggregates, likely dimers and trimers, and have low solubility in toluene and form precipitates. More-complex VOPPs can form large, asphaltene-like nanoaggregates in toluene. Less-functionalized porphyrins are shown to inhibit asphaltene aggregation. More-functionalized porphyrins, potentially having one or more intermolecular interaction sites outside of the central macrocycle, have a tendency to promote aggregation and/or flocculation of asphaltenes at lower temperatures, but have little or no effect at or above room temperature. Highly functionalized porphyrins (petroleum-based and synthetic) are also shown to facilitate asphaltene aggregation, either through the formation of larger nanoaggregates or flocculation, while simpler porphyrins have a tendency to inhibit asphaltene self-assembly.